diff --git a/paddlespeech/s2t/exps/u2/model.py b/paddlespeech/s2t/exps/u2/model.py
index 141e83bce..fdccdf159 100644
--- a/paddlespeech/s2t/exps/u2/model.py
+++ b/paddlespeech/s2t/exps/u2/model.py
@@ -478,7 +478,8 @@ class U2Tester(U2Trainer):
         del input_spec
         infer_model.eval()
 
-        ######################### infer_model.forward_encoder_chunk zero Tensor online ########################
+        ######################### infer_model.forward_encoder_chunk zero Tensor online ############
+        # TODO: 80(feature dim) be configable
         input_spec = [
             paddle.static.InputSpec(shape=[1, None, 80], dtype='float32'),
             paddle.static.InputSpec(shape=[1], dtype='int32'), -1,
@@ -492,6 +493,7 @@ class U2Tester(U2Trainer):
         # paddle.jit.save(static_model, self.args.export_path, combine_params=True)
 
         ######################### infer_model.forward_attention_decoder ########################
+        # TODO: 512(encoder_output) be configable. 1 for B
         input_spec = [
             paddle.static.InputSpec(shape=[None, None], dtype='int64'),
             paddle.static.InputSpec(shape=[None], dtype='int64'),
@@ -501,4 +503,12 @@ class U2Tester(U2Trainer):
             infer_model.forward_attention_decoder, input_spec=input_spec)
         # paddle.jit.save(static_model, self.args.export_path, combine_params=True)
 
+        ######################### infer_model.ctc_activation ########################
+        # TODO: 512(encoder_output) be configable
+        input_spec = [
+            paddle.static.InputSpec(shape=[1, None, 512], dtype='float32')
+        ]
+        infer_model.ctc_activation = paddle.jit.to_static(
+            infer_model.ctc_activation, input_spec=input_spec)
+
         paddle.jit.save(infer_model, './export.jit', combine_params=True)