diff --git a/swift/utils/io_utils.py b/swift/utils/io_utils.py
index 549f0fdde..f355faa6b 100644
--- a/swift/utils/io_utils.py
+++ b/swift/utils/io_utils.py
@@ -56,8 +56,9 @@ def append(self, obj: Union[Dict, List[Dict]]):
         else:
             obj_list = [obj]
         obj_list = check_json_format(obj_list)
-        for _obj in obj_list:
-            self._cache_text += f'{json.dumps(_obj, ensure_ascii=False)}\n'
+        for i, _obj in enumerate(obj_list):
+            obj_list[i] = json.dumps(_obj, ensure_ascii=False) + '\n'
+        self._cache_text = ''.join(obj_list)
 
         if len(self._cache_text) >= self.buffer_size:
             self._write_buffer()