diff --git a/fastNLP/modules/encoder/lstm.py b/fastNLP/modules/encoder/lstm.py
index 1cc0dec1..10d0e339 100644
--- a/fastNLP/modules/encoder/lstm.py
+++ b/fastNLP/modules/encoder/lstm.py
@@ -82,12 +82,12 @@ class LSTM(nn.Module):
             #  解决LSTM无法在DataParallel下使用的问题问题https://github.com/pytorch/pytorch/issues/1591
             if self.batch_first:
                 if output.size(1) < max_len:
-                    dummy_tensor = output.new_zeros(max_len - output.size(1), batch_size, output.size(-1))
-                    output = torch.cat([output, dummy_tensor], 0)
-            else:
-                if output.size(0) < max_len:
                     dummy_tensor = output.new_zeros(batch_size, max_len - output.size(1), output.size(-1))
                     output = torch.cat([output, dummy_tensor], 1)
+            else:
+                if output.size(0) < max_len:
+                    dummy_tensor = output.new_zeros(max_len - output.size(1), batch_size, output.size(-1))
+                    output = torch.cat([output, dummy_tensor], 0)
         else:
             output, hx = self.lstm(x, hx)
         return output, hx