diff --git a/build_docs.py b/build_docs.py
index ffae5be..a744a42 100755
--- a/build_docs.py
+++ b/build_docs.py
@@ -115,6 +115,7 @@ def requirements(self):
 
         return [
             "jieba",  # To improve zh search.
+            "PyStemmer~=2.2.0",  # To improve performance for word stemming.
             "-rrequirements.txt",
         ]