diff --git a/fastNLP/core/trainer.py b/fastNLP/core/trainer.py
index 2a8d85da..b45dd148 100644
--- a/fastNLP/core/trainer.py
+++ b/fastNLP/core/trainer.py
@@ -265,7 +265,7 @@ class Trainer(object):
 
                     # edit prediction
                     self.callback_manager.on_loss_begin(batch_y, prediction)
-                    loss = self._compute_loss(prediction, batch_y)
+                    loss = self._compute_loss(prediction, batch_y).mean()
                     avg_loss += loss.item()
                     loss = loss/self.update_every