diff --git a/fastNLP/modules/encoder/seq2seq_encoder.py b/fastNLP/modules/encoder/seq2seq_encoder.py
index d280582a..5eae1e6d 100644
--- a/fastNLP/modules/encoder/seq2seq_encoder.py
+++ b/fastNLP/modules/encoder/seq2seq_encoder.py
@@ -132,7 +132,7 @@ class TransformerSeq2SeqEncoder(Seq2SeqEncoder):
         x = self.input_fc(x)
         x = F.dropout(x, p=self.dropout, training=self.training)
 
-        encoder_mask = seq_len_to_mask(seq_len)
+        encoder_mask = seq_len_to_mask(seq_len, max_len=max_src_len)
         encoder_mask = encoder_mask.to(device)
 
         for layer in self.layer_stacks: