diff --git a/paddle/fluid/operators/activation_op.cc b/paddle/fluid/operators/activation_op.cc
index 107d333d3a8593e6e3d7afb38c7688d80f2441f8..7dedb36d6e1ef3ed6ebce5be3e53928ee53a984a 100644
--- a/paddle/fluid/operators/activation_op.cc
+++ b/paddle/fluid/operators/activation_op.cc
@@ -242,6 +242,9 @@ $$out = \\left \\lfloor x \\right \\rfloor$$
 UNUSED constexpr char CosDoc[] = R"DOC(
 Cosine Operator. Computes cosine of x element-wise.
 
+Input range is `(-inf, inf)` and output range is `[-1,1]`.
+Return `nan` if input is out of boundary.
+
 $$out = cos(x)$$
 
 )DOC";