diff --git a/mmseg/datasets/custom.py b/mmseg/datasets/custom.py
index c60ef8561..e526b450b 100644
--- a/mmseg/datasets/custom.py
+++ b/mmseg/datasets/custom.py
@@ -233,13 +233,15 @@ class CustomDataset(BaseDataset):
             list[dict]: All data info of dataset.
         """
         data_list = []
+        img_dir = self.data_prefix.get('img_path', None)
         ann_dir = self.data_prefix.get('seg_map_path', None)
         if osp.isfile(self.ann_file):
             lines = mmcv.list_from_file(
                 self.ann_file, file_client_args=self.file_client_args)
             for line in lines:
                 img_name = line.strip()
-                data_info = dict(img_path=img_name + self.img_suffix)
+                data_info = dict(
+                    img_path=osp.join(img_dir, img_name + self.img_suffix))
                 if ann_dir is not None:
                     seg_map = img_name + self.seg_map_suffix
                     data_info['seg_map_path'] = osp.join(ann_dir, seg_map)
@@ -248,7 +250,6 @@ class CustomDataset(BaseDataset):
                 data_info['seg_fields'] = []
                 data_list.append(data_info)
         else:
-            img_dir = self.data_prefix['img_path']
             for img in self.file_client.list_dir_or_file(
                     dir_path=img_dir,
                     list_dir=False,