diff --git a/app/difficulty.py b/app/difficulty.py
index cb93768..1bd8d68 100644
--- a/app/difficulty.py
+++ b/app/difficulty.py
@@ -18,6 +18,7 @@ def load_record(pickle_fname):
     return d
 
 
+ENGLISH_WORD_DIFFICULTY_DICT = {}
 def convert_test_type_to_difficulty_level(d):
     """
     对原本的单词库中的单词进行难度评级
@@ -39,8 +40,10 @@ def convert_test_type_to_difficulty_level(d):
         elif 'BBC' in d[k]:
             result[k] = 8
 
-    return result  # {'apple': 4, ...}
+    global ENGLISH_WORD_DIFFICULTY_DICT
+    ENGLISH_WORD_DIFFICULTY_DICT = result
 
+    return result  # {'apple': 4, ...}
 
 def get_difficulty_level_for_user(d1, d2):
     """
@@ -49,7 +52,11 @@ def get_difficulty_level_for_user(d1, d2):
     在d2的后面添加单词，没有新建一个新的字典
     """
     # TODO: convert_test_type_to_difficulty_level() should not be called every time.  Each word's difficulty level should be pre-computed.
-    d2 = convert_test_type_to_difficulty_level(d2)  # 根据d2的标记评级{'apple': 4, 'abandon': 4, ...}
+    if ENGLISH_WORD_DIFFICULTY_DICT == {}:
+        d2 = convert_test_type_to_difficulty_level(d2)  # 根据d2的标记评级{'apple': 4, 'abandon': 4, ...}
+    else:
+        d2 = ENGLISH_WORD_DIFFICULTY_DICT
+
     stemmer = snowballstemmer.stemmer('english')
 
     for k in d1:  # 用户的词