diff --git a/docs/onnx.md b/docs/onnx.md
new file mode 100644
index 0000000000..3a1d24f311
--- /dev/null
+++ b/docs/onnx.md
@@ -0,0 +1,40 @@
+# Introduction of `onnx` module in MMCV (Experimental)
+
+## register_extra_symbolics
+
+Some extra symbolic functions need to be registered before exporting PyTorch model to ONNX.
+
+### Example
+
+```python
+import mmcv
+from mmcv.onnx import register_extra_symbolics
+
+opset_version = 11
+register_extra_symbolics(opset_version)
+```
+
+## ONNX simplify
+
+### Intention
+
+`mmcv.onnx.simplify` is based on [onnx-simplifier](https://github.com/daquexian/onnx-simplifier), which is a useful tool to make exported ONNX models slimmer by performing a series of optimization. However, for Pytorch models with custom op from `mmcv`, it would break down. Thus, custom ops for ONNX Runtime should be registered.
+
+### Usage
+
+```python
+import onnx
+import numpy as np
+
+import mmcv
+from mmcv.onnx import simplify
+dummy_input = np.random.randn(1, 3, 224, 224).astype(np.float32)
+input = {'input':dummy_input}
+input_file = 'sample.onnx'
+output_file = 'slim.onnx'
+model = simplify(input_file, [input], output_file)
+```
+
+### FAQs
+
+- None
diff --git a/mmcv/onnx/__init__.py b/mmcv/onnx/__init__.py
index 1122c5f866..641999cb06 100644
--- a/mmcv/onnx/__init__.py
+++ b/mmcv/onnx/__init__.py
@@ -1,3 +1,4 @@
+from .simplify import simplify
 from .symbolic import register_extra_symbolics
 
-__all__ = ['register_extra_symbolics']
+__all__ = ['register_extra_symbolics', 'simplify']
diff --git a/mmcv/onnx/simplify/__init__.py b/mmcv/onnx/simplify/__init__.py
new file mode 100644
index 0000000000..d4498ab087
--- /dev/null
+++ b/mmcv/onnx/simplify/__init__.py
@@ -0,0 +1,3 @@
+from .core import simplify
+
+__all__ = ['simplify']
diff --git a/mmcv/onnx/simplify/common.py b/mmcv/onnx/simplify/common.py
new file mode 100644
index 0000000000..6490bd643e
--- /dev/null
+++ b/mmcv/onnx/simplify/common.py
@@ -0,0 +1,43 @@
+import copy
+import warnings
+
+import onnx
+
+
+def add_suffix2name(ori_model, suffix='__', verify=False):
+    """Simplily add a suffix to the name of node, which has a numeric name."""
+    # check if has special op, which has subgraph.
+    special_ops = ('If', 'Loop')
+    for node in ori_model.graph.node:
+        if node.op_type in special_ops:
+            warnings.warn(f'This model has special op: {node.op_type}.')
+            return ori_model
+
+    model = copy.deepcopy(ori_model)
+
+    def need_update(name):
+        return name.isnumeric()
+
+    def update_name(nodes):
+        for node in nodes:
+            if need_update(node.name):
+                node.name += suffix
+
+    update_name(model.graph.initializer)
+    update_name(model.graph.input)
+    update_name(model.graph.output)
+
+    for i, node in enumerate(ori_model.graph.node):
+        # process input of node
+        for j, name in enumerate(node.input):
+            if need_update(name):
+                model.graph.node[i].input[j] = name + suffix
+
+        # process output of node
+        for j, name in enumerate(node.output):
+            if need_update(name):
+                model.graph.node[i].output[j] = name + suffix
+    if verify:
+        onnx.checker.check_model(model)
+
+    return model
diff --git a/mmcv/onnx/simplify/core.py b/mmcv/onnx/simplify/core.py
new file mode 100644
index 0000000000..20428c81ca
--- /dev/null
+++ b/mmcv/onnx/simplify/core.py
@@ -0,0 +1,537 @@
+# This file is modified from https://github.com/daquexian/onnx-simplifier
+import copy
+import os
+from collections import OrderedDict
+from typing import Dict, List, Optional, Sequence, Union
+
+import numpy as np  # type: ignore
+import onnx  # type: ignore
+import onnx.helper  # type: ignore
+import onnx.numpy_helper
+import onnx.shape_inference  # type: ignore
+import onnxoptimizer  # type: ignore
+import onnxruntime as rt  # type: ignore
+
+from .common import add_suffix2name
+
+TensorShape = List[int]
+TensorShapes = Dict[Optional[str], TensorShape]
+
+
+def add_features_to_output(m: onnx.ModelProto,
+                           nodes: List[onnx.NodeProto]) -> None:
+    """Add features to output in pb, so that ONNX Runtime will output them.
+
+    Args:
+        m (onnx.ModelProto): Input ONNX model.
+        nodes (List[onnx.NodeProto]): List of ONNX nodes, whose outputs
+        will be added into the graph output.
+    """
+    for node in nodes:
+        for output in node.output:
+            m.graph.output.extend([onnx.ValueInfoProto(name=output)])
+
+
+def get_shape_from_value_info_proto(v: onnx.ValueInfoProto) -> List[int]:
+    return [dim.dim_value for dim in v.type.tensor_type.shape.dim]
+
+
+def get_value_info_all(m: onnx.ModelProto,
+                       name: str) -> Optional[onnx.ValueInfoProto]:
+    for v in m.graph.value_info:
+        if v.name == name:
+            return v
+
+    for v in m.graph.input:
+        if v.name == name:
+            return v
+
+    for v in m.graph.output:
+        if v.name == name:
+            return v
+
+    return None
+
+
+def get_shape(m: onnx.ModelProto, name: str) -> TensorShape:
+    """Get shape info of a node in a model.
+
+    Args:
+        m (onnx.ModelProto): Input model.
+        name (str): Name of a node.
+
+    Returns:
+        TensorShape: Shape of a node.
+
+    Note:
+        This method relies on onnx shape inference, which is not reliable.
+        So only use it on input or output tensors
+    """
+    v = get_value_info_all(m, name)
+    if v is not None:
+        return get_shape_from_value_info_proto(v)
+    raise RuntimeError('Cannot get shape of "{}"'.format(name))
+
+
+def get_elem_type(m: onnx.ModelProto, name: str) -> Optional[int]:
+    v = get_value_info_all(m, name)
+    if v is not None:
+        return v.type.tensor_type.elem_type
+    return None
+
+
+def get_np_type_from_elem_type(elem_type: int) -> int:
+    """Map element type from ONNX to dtype of numpy.
+
+    Args:
+        elem_type (int): Element type index in ONNX.
+
+    Returns:
+        int: Data type in numpy.
+    """
+    # from https://github.com/onnx/onnx/blob/e5e9a539f550f07ec156812484e8d4f33fb91f88/onnx/onnx.proto#L461 # noqa: E501
+    sizes = (None, np.float32, np.uint8, np.int8, np.uint16, np.int16,
+             np.int32, np.int64, str, np.bool, np.float16, np.double,
+             np.uint32, np.uint64, np.complex64, np.complex128, np.float16)
+    assert len(sizes) == 17
+    size = sizes[elem_type]
+    assert size is not None
+    return size
+
+
+def get_input_names(model: onnx.ModelProto) -> List[str]:
+    """Get input names of a model.
+
+    Args:
+        model (onnx.ModelProto): Input ONNX model.
+
+    Returns:
+        List[str]: List of input names.
+    """
+    input_names = list(
+        set([ipt.name for ipt in model.graph.input]) -
+        set([x.name for x in model.graph.initializer]))
+    return input_names
+
+
+def add_initializers_into_inputs(model: onnx.ModelProto) -> onnx.ModelProto:
+    """add initializers into inputs of a model.
+
+    Args:
+        model (onnx.ModelProto): Input ONNX model.
+
+    Returns:
+        onnx.ModelProto: Updated ONNX model.
+    """
+    for x in model.graph.initializer:
+        input_names = [x.name for x in model.graph.input]
+        if x.name not in input_names:
+            shape = onnx.TensorShapeProto()
+            for dim in x.dims:
+                shape.dim.extend(
+                    [onnx.TensorShapeProto.Dimension(dim_value=dim)])
+            model.graph.input.extend([
+                onnx.ValueInfoProto(
+                    name=x.name,
+                    type=onnx.TypeProto(
+                        tensor_type=onnx.TypeProto.Tensor(
+                            elem_type=x.data_type, shape=shape)))
+            ])
+    return model
+
+
+def generate_rand_input(
+        model: onnx.ModelProto,
+        input_shapes: Optional[TensorShapes] = None) -> Dict[str, np.ndarray]:
+    """Generate random input for a model.
+
+    Args:
+        model (onnx.ModelProto): Input ONNX model.
+        input_shapes (TensorShapes, optional): Input shapes of the model.
+
+    Returns:
+        Dict[str, np.ndarray]: Generated inputs of `np.ndarray`.
+    """
+    if input_shapes is None:
+        input_shapes = {}
+    input_names = get_input_names(model)
+    full_input_shapes = {ipt: get_shape(model, ipt) for ipt in input_names}
+    assert None not in input_shapes
+    full_input_shapes.update(input_shapes)  # type: ignore
+    for key in full_input_shapes:
+        if np.prod(full_input_shapes[key]) <= 0:
+            raise RuntimeError(f'The shape of input "{key}" has dynamic size, \
+                please determine the input size manually.')
+
+    inputs = {
+        ipt: np.array(
+            np.random.rand(*full_input_shapes[ipt]),
+            dtype=get_np_type_from_elem_type(get_elem_type(model, ipt)))
+        for ipt in input_names
+    }
+    return inputs
+
+
+def get_constant_nodes(m: onnx.ModelProto) -> List[onnx.NodeProto]:
+    """Collect constant nodes from a model.
+
+    Args:
+        m (onnx.ModelProto): Input ONNX model.
+
+    Returns:
+        List[onnx.NodeProto]: List of constant nodes.
+    """
+
+    const_nodes = []
+    const_tensors = [x.name for x in m.graph.initializer]
+    const_tensors.extend([
+        node.output[0] for node in m.graph.node if node.op_type == 'Constant'
+    ])
+    # The output shape of some node types is determined by the input value
+    # we consider the output of this node doesn't have constant shape,
+    # so we do not simplify a such node even if the node is Shape op
+    dynamic_tensors = []
+
+    def is_dynamic(node):
+        if node.op_type in ['NonMaxSuppression', 'NonZero', 'Unique'
+                            ] and node.input[0] not in const_tensors:
+            return True
+        if node.op_type in [
+                'Reshape', 'Expand', 'Upsample', 'ConstantOfShape'
+        ] and len(node.input) > 1 and node.input[1] not in const_tensors:
+            return True
+        if node.op_type in ['Resize'] and (
+            (len(node.input) > 2 and node.input[2] not in const_tensors) or
+            (len(node.input) > 3
+             and node.input[3] not in const_tensors)):  # noqa: E129
+            return True
+        return False
+
+    for node in m.graph.node:
+        if any(x in dynamic_tensors for x in node.input):
+            dynamic_tensors.extend(node.output)
+        elif node.op_type == 'Shape':
+            const_nodes.append(node)
+            const_tensors.extend(node.output)
+        elif is_dynamic(node):
+            dynamic_tensors.extend(node.output)
+        elif all([x in const_tensors for x in node.input]):
+            const_nodes.append(node)
+            const_tensors.extend(node.output)
+    return copy.deepcopy(const_nodes)
+
+
+def forward(
+        model: onnx.ModelProto,
+        inputs: Dict[str, np.ndarray] = None,
+        input_shapes: Optional[TensorShapes] = None) -> Dict[str, np.ndarray]:
+    """Run forward on a model.
+
+    Args:
+        model (onnx.ModelProto): Input ONNX model.
+        inputs (Dict[str, np.ndarray], optional): Inputs of the model.
+        input_shapes (TensorShapes, optional): Input shapes of the model.
+
+    Returns:
+        Dict[str, np.ndarray]: Outputs of the model.
+    """
+    if input_shapes is None:
+        input_shapes = {}
+    sess_options = rt.SessionOptions()
+    # load custom lib for onnxruntime in mmcv
+    ort_custom_op_path = ''
+    try:
+        from mmcv.ops import get_onnxruntime_op_path
+        ort_custom_op_path = get_onnxruntime_op_path()
+    except ImportError:
+        pass
+    if os.path.exists(ort_custom_op_path):
+        sess_options.register_custom_ops_library(ort_custom_op_path)
+    sess_options.graph_optimization_level = rt.GraphOptimizationLevel(0)
+    sess_options.log_severity_level = 3
+    sess = rt.InferenceSession(
+        model.SerializeToString(),
+        sess_options=sess_options,
+        providers=['CPUExecutionProvider'])
+    if inputs is None:
+        inputs = generate_rand_input(model, input_shapes=input_shapes)
+    outputs = [x.name for x in sess.get_outputs()]
+    run_options = rt.RunOptions()
+    run_options.log_severity_level = 3
+    res = OrderedDict(
+        zip(outputs, sess.run(outputs, inputs, run_options=run_options)))
+    return res
+
+
+def forward_for_node_outputs(
+        model: onnx.ModelProto,
+        nodes: List[onnx.NodeProto],
+        input_shapes: Optional[TensorShapes] = None,
+        inputs: Optional[Dict[str,
+                              np.ndarray]] = None) -> Dict[str, np.ndarray]:
+    if input_shapes is None:
+        input_shapes = {}
+    model = copy.deepcopy(model)
+    add_features_to_output(model, nodes)
+    res = forward(model, inputs=inputs, input_shapes=input_shapes)
+    return res
+
+
+def insert_elem(repeated_container, index: int, element):
+    repeated_container.extend([repeated_container[-1]])
+    for i in reversed(range(index + 1, len(repeated_container) - 1)):
+        repeated_container[i].CopyFrom(repeated_container[i - 1])
+    repeated_container[index].CopyFrom(element)
+
+
+def eliminate_const_nodes(model: onnx.ModelProto,
+                          const_nodes: List[onnx.NodeProto],
+                          res: Dict[str, np.ndarray]) -> onnx.ModelProto:
+    """Eliminate redundant constant nodes from model.
+
+    Args:
+        model (onnx.ModelProto): The original ONNX model.
+        const_nodes (List[onnx.NodeProto]):
+            Constant nodes detected by `get_constant_nodes`.
+        res (Dict[str, np.ndarray]): Outputs of the model.
+
+    Returns:
+        onnx.ModelProto: The simplified onnx model.
+    """
+
+    for i, node in enumerate(model.graph.node):
+        if node in const_nodes:
+            for output in node.output:
+                new_node = copy.deepcopy(node)
+                new_node.name = 'node_' + output
+                new_node.op_type = 'Constant'
+                new_attr = onnx.helper.make_attribute(
+                    'value',
+                    onnx.numpy_helper.from_array(res[output], name=output))
+                del new_node.input[:]
+                del new_node.attribute[:]
+                del new_node.output[:]
+                new_node.output.extend([output])
+                new_node.attribute.extend([new_attr])
+                insert_elem(model.graph.node, i + 1, new_node)
+            del model.graph.node[i]
+
+    return model
+
+
+def optimize(model: onnx.ModelProto, skip_fuse_bn: bool,
+             skipped_optimizers: Optional[Sequence[str]]) -> onnx.ModelProto:
+    """Perform optimization on an ONNX model. Before simplifying, use this
+    method to generate value_info. After simplifying, use this method to fold
+    constants generated in previous step into initializer, and eliminate unused
+    constants.
+
+    Args:
+        model (onnx.ModelProto): The input ONNX model.
+        skip_fuse_bn (bool): Whether to skip fuse bn.
+        skipped_optimizers (Sequence[str]): List of optimizers to be skipped.
+
+    Returns:
+        onnx.ModelProto: The optimized model.
+    """
+    # Due to a onnx bug, https://github.com/onnx/onnx/issues/2417,
+    # we need to add missing initializers into inputs
+    onnx.checker.check_model(model)
+    input_num = len(model.graph.input)
+    model = add_initializers_into_inputs(model)
+    onnx.helper.strip_doc_string(model)
+    onnx.checker.check_model(model)
+    optimizers_list = [
+        'eliminate_deadend', 'eliminate_nop_dropout', 'eliminate_nop_cast',
+        'eliminate_nop_monotone_argmax', 'eliminate_nop_pad',
+        'extract_constant_to_initializer', 'eliminate_unused_initializer',
+        'eliminate_nop_transpose', 'eliminate_identity',
+        'fuse_add_bias_into_conv', 'fuse_consecutive_concats',
+        'fuse_consecutive_log_softmax', 'fuse_consecutive_reduce_unsqueeze',
+        'fuse_consecutive_squeezes', 'fuse_consecutive_transposes',
+        'fuse_matmul_add_bias_into_gemm', 'fuse_pad_into_conv',
+        'fuse_transpose_into_gemm'
+    ]
+    if not skip_fuse_bn:
+        optimizers_list.append('fuse_bn_into_conv')
+    if skipped_optimizers is not None:
+        for opt in skipped_optimizers:
+            try:
+                optimizers_list.remove(opt)
+            except ValueError:
+                pass
+
+    model = onnxoptimizer.optimize(model, optimizers_list, fixed_point=True)
+    if model.ir_version > 3:
+        del model.graph.input[input_num:]
+    onnx.checker.check_model(model)
+    return model
+
+
+def check(model_opt: onnx.ModelProto,
+          model_ori: onnx.ModelProto,
+          n_times: int = 5,
+          input_shapes: Optional[TensorShapes] = None,
+          inputs: Optional[List[Dict[str, np.ndarray]]] = None) -> bool:
+    """Check model before and after simplify.
+
+    Args:
+        model_opt (onnx.ModelProto): Optimized model.
+        model_ori (onnx.ModelProto): Original model.
+        n_times (int, optional): Number of times to compare models.
+        input_shapes (TensorShapes, optional): Input shapes of the model.
+        inputs (List[Dict[str, np.ndarray]], optional): Inputs of the model.
+
+    Returns:
+        bool: `True` means the outputs of two models have neglectable
+            numeric difference.
+    """
+
+    if input_shapes is None:
+        input_shapes = {}
+    onnx.checker.check_model(model_opt)
+    if inputs is not None:
+        n_times = min(n_times, len(inputs))
+    for i in range(n_times):
+        print(f'Checking {i}/{n_times}...')
+        if inputs is None:
+            model_input = generate_rand_input(
+                model_opt, input_shapes=input_shapes)
+        else:
+            model_input = inputs[i]
+        res_opt = forward(model_opt, inputs=model_input)
+        res_ori = forward(model_ori, inputs=model_input)
+
+        for name in res_opt.keys():
+            if not np.allclose(
+                    res_opt[name], res_ori[name], rtol=1e-4, atol=1e-5):
+                print(
+                    'Tensor {} changes after simplifying. The max diff is {}.'.
+                    format(name,
+                           np.max(np.abs(res_opt[name] - res_ori[name]))))
+                print('Note that the checking is not always correct.')
+                print('After simplifying:')
+                print(res_opt[name])
+                print('Before simplifying:')
+                print(res_ori[name])
+                print('----------------')
+                return False
+    return True
+
+
+def clean_constant_nodes(const_nodes: List[onnx.NodeProto],
+                         res: Dict[str, np.ndarray]):
+    """Clean constant nodes.
+
+    Args:
+        const_nodes (List[onnx.NodeProto]): List of constant nodes.
+        res (Dict[str, np.ndarray]): The forward result of model.
+
+    Returns:
+        List[onnx.NodeProto]:  The constant nodes which have an output in res.
+
+    Notes:
+        It seems not needed since commit 6f2a72, but maybe it still prevents
+        some unknown bug.
+    """
+
+    return [node for node in const_nodes if node.output[0] in res]
+
+
+def check_and_update_input_shapes(model: onnx.ModelProto,
+                                  input_shapes: TensorShapes) -> TensorShapes:
+    input_names = get_input_names(model)
+    if None in input_shapes:
+        if len(input_names) == 1:
+            input_shapes[input_names[0]] = input_shapes[None]
+            del input_shapes[None]
+        else:
+            raise RuntimeError('The model has more than 1 inputs!')
+    for x in input_shapes:
+        if x not in input_names:
+            raise RuntimeError(f'The model doesn\'t have input named "{x}"')
+    return input_shapes
+
+
+def simplify(model: Union[str, onnx.ModelProto],
+             inputs: Sequence[Dict[str, np.ndarray]] = None,
+             output_file: str = None,
+             perform_optimization: bool = True,
+             skip_fuse_bn: bool = False,
+             skip_shape_inference: bool = True,
+             input_shapes: Dict[str, Sequence[int]] = None,
+             skipped_optimizers: Sequence[str] = None) -> onnx.ModelProto:
+    """Simplify and optimize an onnx model.
+
+    For models from detection and segmentation, it is strongly suggested to
+    input multiple input images for verification.
+
+    Arguments:
+        model (str or onnx.ModelProto): path of model or loaded model object.
+        inputs (optional, Sequence[Dict[str, np.ndarray]]): inputs of model.
+        output_file (optional, str): output file to save simplified model.
+        perform_optimization (optional, bool): whether to perform optimization.
+        skip_fuse_bn (optional, bool): whether to skip fusing bn layer.
+        skip_shape_inference (optional, bool): whether to skip shape inference.
+        input_shapes (optional, Dict[str, Sequence[int]]):
+            the shapes of model inputs.
+        skipped_optimizers (optional, Sequence[str]):
+            the names of optimizer to be skipped.
+
+    Returns:
+        onnx.ModelProto: simplified and optimized onnx model.
+
+    Example:
+        >>> import onnx
+        >>> import numpy as np
+        >>>
+        >>> from mmcv.onnx import simplify
+        >>>
+        >>> dummy_input = np.random.randn(1, 3, 224, 224).astype(np.float32)
+        >>> input = {'input':dummy_input}
+        >>> input_file = 'sample.onnx'
+        >>> output_file = 'slim.onnx'
+        >>> model = simplify(input_file, [input], output_file)
+    """
+    if input_shapes is None:
+        input_shapes = {}
+    if isinstance(model, str):
+        model = onnx.load(model)
+    # rename op with numeric name for issue
+    # https://github.com/onnx/onnx/issues/2613
+    model = add_suffix2name(model)
+    onnx.checker.check_model(model)
+    model_ori = copy.deepcopy(model)
+    numel_node_ori = len(model_ori.graph.node)
+    if not skip_shape_inference:
+        model = onnx.shape_inference.infer_shapes(model)
+
+    input_shapes = check_and_update_input_shapes(model, input_shapes)
+
+    if perform_optimization:
+        model = optimize(model, skip_fuse_bn, skipped_optimizers)
+
+    const_nodes = get_constant_nodes(model)
+    feed_inputs = None if inputs is None else inputs[0]
+    res = forward_for_node_outputs(
+        model, const_nodes, input_shapes=input_shapes, inputs=feed_inputs)
+    const_nodes = clean_constant_nodes(const_nodes, res)
+    model = eliminate_const_nodes(model, const_nodes, res)
+    onnx.checker.check_model(model)
+
+    if perform_optimization:
+        model = optimize(model, skip_fuse_bn, skipped_optimizers)
+
+    check_ok = check(
+        model_ori, model, input_shapes=input_shapes, inputs=inputs)
+
+    assert check_ok, 'Check failed for the simplified model!'
+    numel_node_slim = len(model.graph.node)
+    print(f'Number of nodes: {numel_node_ori} -> {numel_node_slim}')
+
+    if output_file is not None:
+        save_dir, _ = os.path.split(output_file)
+        if save_dir:
+            os.makedirs(save_dir, exist_ok=True)
+        onnx.save(model, output_file)
+    return model
diff --git a/requirements/test.txt b/requirements/test.txt
index e26c846137..bec4fabd1c 100644
--- a/requirements/test.txt
+++ b/requirements/test.txt
@@ -1,6 +1,7 @@
 coverage
 lmdb
 onnx==1.7.0
+onnxoptimizer
 onnxruntime==1.4.0
 pytest
 PyTurboJPEG
diff --git a/setup.cfg b/setup.cfg
index 3406d49daa..1ef231612f 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -14,6 +14,6 @@ line_length = 79
 multi_line_output = 0
 known_standard_library = pkg_resources,setuptools
 known_first_party = mmcv
-known_third_party = addict,cv2,m2r,numpy,onnx,onnxruntime,pytest,recommonmark,resnet_cifar,torch,torchvision,yaml,yapf
+known_third_party = addict,cv2,m2r,numpy,onnx,onnxoptimizer,onnxruntime,packaging,pytest,recommonmark,resnet_cifar,torch,torchvision,yaml,yapf
 no_lines_before = STDLIB,LOCALFOLDER
 default_section = THIRDPARTY
diff --git a/tests/test_ops/test_onnx.py b/tests/test_ops/test_onnx.py
index 1021d18adc..d70e2fb677 100644
--- a/tests/test_ops/test_onnx.py
+++ b/tests/test_ops/test_onnx.py
@@ -8,6 +8,7 @@
 import pytest
 import torch
 import torch.nn as nn
+from packaging import version
 
 onnx_file = 'tmp.onnx'
 
@@ -65,7 +66,6 @@ def test_nms():
     reason='CUDA is unavailable for test_softnms')
 def test_softnms():
     from mmcv.ops import get_onnxruntime_op_path, soft_nms
-    from packaging import version
 
     # only support pytorch >= 1.7.0
     if version.parse(torch.__version__) < version.parse('1.7.0'):
@@ -270,3 +270,27 @@ def warpped_function(torch_input, torch_rois):
         # allclose
         os.remove(onnx_file)
         assert np.allclose(pytorch_output, onnx_output, atol=1e-3)
+
+
+def test_simplify():
+    from mmcv.onnx import simplify
+
+    # only support PyTorch >= 1.5.0
+    if version.parse(torch.__version__) < version.parse('1.5.0'):
+        pytest.skip('mmcv.onnx.simplify only support with PyTorch >= 1.5.0')
+
+    def foo(x):
+        y = x.view((x.shape[0], x.shape[1], x.shape[3], x.shape[2]))
+        return y
+
+    net = WrapFunction(foo)
+    dummy_input = torch.randn(2, 3, 4, 5)
+    torch.onnx.export(net, dummy_input, onnx_file, input_names=['input'])
+    ori_onnx_model = onnx.load(onnx_file)
+
+    feed_input = [{'input': dummy_input.detach().cpu().numpy()}]
+    slim_onnx_model = simplify(ori_onnx_model, feed_input, onnx_file)
+    numel_before = len(ori_onnx_model.graph.node)
+    numel_after = len(slim_onnx_model.graph.node)
+    os.remove(onnx_file)
+    assert numel_before == 18 and numel_after == 1, 'Simplify failed.'