diff --git a/tools/train.py b/tools/train.py
index d14a3d6fba1..41ea55d3b4c 100644
--- a/tools/train.py
+++ b/tools/train.py
@@ -3,6 +3,7 @@
 import copy
 import os
 import os.path as osp
+import string
 import time
 import warnings
 
@@ -98,6 +99,17 @@ def main():
     args = parse_args()
 
     cfg = Config.fromfile(args.config)
+
+    # update data root according to environment variable
+    if os.environ.get('MMDET_DATASETS', None) is not None:
+        def update_data_root(cfg, str_o, str_n):
+            for k, v in cfg.items():
+                if isinstance(v, mmcv.ConfigDict):
+                    update_data_root(cfg[k], str_o, str_n)
+                if isinstance(v, str):
+                    cfg[k] = v.replace(str_o, str_n)
+        update_data_root(cfg, cfg.data_root, os.environ['MMDET_DATASETS'])
+
     if args.cfg_options is not None:
         cfg.merge_from_dict(args.cfg_options)
 
@@ -108,10 +120,6 @@ def main():
     if cfg.get('cudnn_benchmark', False):
         torch.backends.cudnn.benchmark = True
 
-    # update data root according to environment variable
-    if os.environ.get('MMDET_DATASETS', None) is not None:
-        cfg.data_root = os.environ['MMDET_DATASETS']
-
     # work_dir is determined in this priority: CLI > segment in file > filename
     if args.work_dir is not None:
         # update configs according to CLI args if args.work_dir is not None