diff --git a/fastNLP/core/vocabulary.py b/fastNLP/core/vocabulary.py
index 7b0ab614..ca6b4ebf 100644
--- a/fastNLP/core/vocabulary.py
+++ b/fastNLP/core/vocabulary.py
@@ -16,14 +16,35 @@ def isiterable(p_object):
 
 
 def check_build_vocab(func):
+    """A decorator to make sure the indexing is built before used.
+
+    """
+
     def _wrapper(self, *args, **kwargs):
-        if self.word2idx is None:
+        if self.word2idx is None or self.rebuild is True:
             self.build_vocab()
         return func(self, *args, **kwargs)
 
     return _wrapper
 
 
+def check_build_status(func):
+    """A decorator to check whether the vocabulary updates after the last build.
+
+    """
+
+    def _wrapper(self, *args, **kwargs):
+        if self.rebuild is False:
+            self.rebuild = True
+            if self.max_size is not None and len(self.word_count) >= self.max_size:
+                print("[Warning] Vocabulary has reached the max size {} when calling {} method. "
+                      "Adding more words may cause unexpected behaviour of Vocabulary. ".format(
+                    self.max_size, func.__name__))
+        return func(self, *args, **kwargs)
+
+    return _wrapper
+
+
 class Vocabulary(object):
     """Use for word and index one to one mapping
 
@@ -54,7 +75,9 @@ class Vocabulary(object):
             self.unknown_label = None
         self.word2idx = None
         self.idx2word = None
+        self.rebuild = True
 
+    @check_build_status
     def update(self, word_lst):
         """Add a list of words into the vocabulary.
 
@@ -62,6 +85,7 @@ class Vocabulary(object):
         """
         self.word_count.update(word_lst)
 
+    @check_build_status
     def add(self, word):
         """Add a single word into the vocabulary.
 
@@ -69,6 +93,7 @@ class Vocabulary(object):
         """
         self.word_count[word] += 1
 
+    @check_build_status
     def add_word(self, word):
         """Add a single word into the vocabulary.
 
@@ -76,6 +101,7 @@ class Vocabulary(object):
         """
         self.add(word)
 
+    @check_build_status
     def add_word_lst(self, word_lst):
         """Add a list of words into the vocabulary.
 
@@ -101,6 +127,7 @@ class Vocabulary(object):
         start_idx = len(self.word2idx)
         self.word2idx.update({w: i + start_idx for i, (w, _) in enumerate(words)})
         self.build_reverse_vocab()
+        self.rebuild = False
 
     def build_reverse_vocab(self):
         """Build 'index to word' dict based on 'word to index' dict.
@@ -188,4 +215,3 @@ class Vocabulary(object):
         """
         self.__dict__.update(state)
         self.build_reverse_vocab()
-
diff --git a/test/core/test_vocabulary.py b/test/core/test_vocabulary.py
index e140b1aa..e453e935 100644
--- a/test/core/test_vocabulary.py
+++ b/test/core/test_vocabulary.py
@@ -59,3 +59,30 @@ class TestIndexing(unittest.TestCase):
         vocab = Vocabulary(need_default=True, max_size=None, min_freq=None)
         vocab.update(text)
         self.assertEqual(text, [vocab.to_word(idx) for idx in [vocab[w] for w in text]])
+
+
+class TestOther(unittest.TestCase):
+    def test_additional_update(self):
+        vocab = Vocabulary(need_default=True, max_size=None, min_freq=None)
+        vocab.update(text)
+
+        _ = vocab["well"]
+        self.assertEqual(vocab.rebuild, False)
+
+        vocab.add("hahaha")
+        self.assertEqual(vocab.rebuild, True)
+
+        _ = vocab["hahaha"]
+        self.assertEqual(vocab.rebuild, False)
+        self.assertTrue("hahaha" in vocab)
+
+    def test_warning(self):
+        vocab = Vocabulary(need_default=True, max_size=len(set(text)), min_freq=None)
+        vocab.update(text)
+        self.assertEqual(vocab.rebuild, True)
+        print(len(vocab))
+        self.assertEqual(vocab.rebuild, False)
+
+        vocab.update(["hahahha", "hhh", "vvvv", "ass", "asss", "jfweiong", "eqgfeg", "feqfw"])
+        # this will print a warning
+        self.assertEqual(vocab.rebuild, True)