diff --git a/ppsci/data/__init__.py b/ppsci/data/__init__.py
index 0faea3beb..5e2b36057 100644
--- a/ppsci/data/__init__.py
+++ b/ppsci/data/__init__.py
@@ -83,7 +83,7 @@ def build_dataloader(_dataset, cfg):
     # build collate_fn if specified
     batch_transforms_cfg = cfg.pop("batch_transforms", None)
 
-    collate_fn = batch_transform.default_collate_fn
+    collate_fn = None
     if isinstance(batch_transforms_cfg, (list, tuple)):
         collate_fn = batch_transform.build_batch_transforms(batch_transforms_cfg)
 
@@ -98,8 +98,13 @@ def build_dataloader(_dataset, cfg):
     # build dataloader
     if getattr(_dataset, "use_pgl", False):
         # Use special dataloader from "Paddle Graph Learning" toolkit.
-        from pgl.utils import data as pgl_data
+        try:
+            from pgl.utils import data as pgl_data
+        except ModuleNotFoundError:
+            logger.error("Please install pgl with `pip install pgl`.")
+            raise ModuleNotFoundError("pgl is not installed.")
 
+        collate_fn = batch_transform.default_collate_fn
         dataloader_ = pgl_data.Dataloader(
             dataset=_dataset,
             batch_size=cfg["batch_size"],
diff --git a/ppsci/data/process/batch_transform/__init__.py b/ppsci/data/process/batch_transform/__init__.py
index badee27b7..e45279bda 100644
--- a/ppsci/data/process/batch_transform/__init__.py
+++ b/ppsci/data/process/batch_transform/__init__.py
@@ -46,20 +46,6 @@ def default_collate_fn(batch: List[Any]) -> Any:
     sample = batch[0]
     if sample is None:
         return None
-    elif isinstance(sample, pgl.Graph):
-        graph = pgl.Graph(
-            num_nodes=sample.num_nodes,
-            edges=sample.edges,
-        )
-        graph.x = paddle.concat([g.x for g in batch])
-        graph.y = paddle.concat([g.y for g in batch])
-        graph.edge_index = paddle.concat([g.edge_index for g in batch], axis=1)
-        graph.edge_attr = paddle.concat([g.edge_attr for g in batch])
-        graph.pos = paddle.concat([g.pos for g in batch])
-        graph.shape = [
-            len(batch),
-        ]
-        return graph
     elif isinstance(sample, np.ndarray):
         batch = np.stack(batch, axis=0)
         return batch
@@ -77,10 +63,20 @@ def default_collate_fn(batch: List[Any]) -> Any:
         if not all(len(sample) == sample_fields_num for sample in iter(batch)):
             raise RuntimeError("fileds number not same among samples in a batch")
         return [default_collate_fn(fields) for fields in zip(*batch)]
+    elif str(type(sample)) == "<class 'pgl.graph.Graph'>":
+        # use str(type()) instead of isinstance() in case of pgl is not installed.
+        graph = pgl.Graph(num_nodes=sample.num_nodes, edges=sample.edges)
+        graph.x = paddle.concat([g.x for g in batch])
+        graph.y = paddle.concat([g.y for g in batch])
+        graph.edge_index = paddle.concat([g.edge_index for g in batch], axis=1)
+        graph.edge_attr = paddle.concat([g.edge_attr for g in batch])
+        graph.pos = paddle.concat([g.pos for g in batch])
+        graph.shape = [len(batch)]
+        return graph
 
     raise TypeError(
-        "batch data can only contains: tensor, numpy.ndarray, "
-        f"dict, list, number, None, but got {type(sample)}"
+        "batch data can only contains: Tensor, numpy.ndarray, "
+        f"dict, list, number, None, pgl.Graph, but got {type(sample)}"
     )