diff --git a/python/paddle/trainer_config_helpers/activations.py b/python/paddle/trainer_config_helpers/activations.py
index 00efc01c0592107314f5b23c951706d039d49a88..3683968262266a2d654d2480b828173bc761152b 100644
--- a/python/paddle/trainer_config_helpers/activations.py
+++ b/python/paddle/trainer_config_helpers/activations.py
@@ -77,7 +77,7 @@ class SoftmaxActivation(BaseActivation):
 
     .. math::
 
-       P(y=j|x) = \\frac{e^{x_j}} {\\sum^K_{k=1} e^{x_j} }
+       P(y=j|x) = \\frac{e^{x_j}} {\\sum^K_{k=1} e^{x_k} }
     """
 
     def __init__(self):