diff --git a/fastNLP/core/fieldarray.py b/fastNLP/core/fieldarray.py
index 3a63f788..f93fbf2e 100644
--- a/fastNLP/core/fieldarray.py
+++ b/fastNLP/core/fieldarray.py
@@ -6,6 +6,7 @@ class FieldArray(object):
     It is the basic element of DataSet class.
 
     """
+
     def __init__(self, name, content, padding_val=0, is_target=False, is_input=False):
         """
 
@@ -20,21 +21,56 @@ class FieldArray(object):
         self.padding_val = padding_val
         self.is_target = is_target
         self.is_input = is_input
-        # TODO: auto detect dtype
-        self.dtype = None
+        self.pytype = self._type_detection(content)
+        self.dtype = self._map_to_np_type(self.pytype)
+
+    @staticmethod
+    def _type_detection(content):
+        type_set = set([type(item) for item in content])
+        if len(type_set) == 1 and any(basic_type in type_set for basic_type in (str, int, float)):
+            return type_set.pop()
+        elif len(type_set) == 2 and float in type_set and int in type_set:
+            # up-cast int to float
+            for idx, _ in enumerate(content):
+                content[idx] = float(content[idx])
+            return float
+        else:
+            raise ValueError("Unsupported type conversion detected in FieldArray: {}".format(*type_set))
+
+    @staticmethod
+    def _map_to_np_type(basic_type):
+        type_mapping = {int: np.int64, float: np.double, str: np.str}
+        return type_mapping[basic_type]
 
     def __repr__(self):
         return "FieldArray {}: {}".format(self.name, self.content.__repr__())
 
     def append(self, val):
+        """Add a new item to the tail of FieldArray.
+
+        :param val: int, float, or str.
+        """
+        val_type = type(val)
+        if val_type is int and self.pytype is float:
+            # up-cast the appended value
+            val = float(val)
+        elif val_type is float and self.pytype is int:
+            # up-cast all other values in the content
+            for idx, _ in enumerate(self.content):
+                self.content[idx] = float(self.content[idx])
+            self.pytype = float
+            self.dtype = self._map_to_np_type(self.pytype)
+
+        elif val_type != self.pytype:
+            raise ValueError("Cannot append a {}-type value into a {}-tpye FieldArray.".format(val_type, self.pytype))
         self.content.append(val)
 
-    def __getitem__(self, name):
-        return self.get(name)
+    def __getitem__(self, indices):
+        return self.get(indices)
 
-    def __setitem__(self, name, val):
-        assert isinstance(name, int)
-        self.content[name] = val
+    def __setitem__(self, idx, val):
+        assert isinstance(idx, int)
+        self.content[idx] = val
 
     def get(self, indices):
         """Fetch instances based on indices.
@@ -42,31 +78,32 @@ class FieldArray(object):
         :param indices: an int, or a list of int.
         :return:
         """
+        # TODO: 返回行为不一致，有隐患
         if isinstance(indices, int):
             return self.content[indices]
         assert self.is_input is True or self.is_target is True
         batch_size = len(indices)
         # TODO 当这个fieldArray是seq_length这种只有一位的内容时，不需要padding，需要再讨论一下
-        if not isiterable(self.content[0]):
-            if self.dtype is None:
-                self.dtype = np.int64 if isinstance(self.content[0], int) else np.double
+        if not is_iterable(self.content[0]):
             array = np.array([self.content[i] for i in indices], dtype=self.dtype)
         else:
-            if self.dtype is None:
-                self.dtype = np.int64
             max_len = max([len(self.content[i]) for i in indices])
             array = np.full((batch_size, max_len), self.padding_val, dtype=self.dtype)
-
             for i, idx in enumerate(indices):
                 array[i][:len(self.content[idx])] = self.content[idx]
         return array
 
     def __len__(self):
+        """Returns the size of FieldArray.
+
+        :return int length:
+        """
         return len(self.content)
 
-def isiterable(content):
+
+def is_iterable(content):
     try:
         _ = (e for e in content)
     except TypeError:
         return False
-    return True
\ No newline at end of file
+    return True
diff --git a/test/core/test_fieldarray.py b/test/core/test_fieldarray.py
index 07f02c54..883e1136 100644
--- a/test/core/test_fieldarray.py
+++ b/test/core/test_fieldarray.py
@@ -20,3 +20,23 @@ class TestFieldArray(unittest.TestCase):
         self.assertEqual(fa.get(0), 1)
         self.assertTrue(isinstance(fa.get([0, 1, 2]), np.ndarray))
         self.assertListEqual(list(fa.get([0, 1, 2])), [1, 2, 3])
+
+    def test_type_conversion(self):
+        fa = FieldArray("x", [1.2, 2.2, 3, 4, 5], is_input=True)
+        self.assertEqual(fa.pytype, float)
+        self.assertEqual(fa.dtype, np.double)
+
+        fa = FieldArray("x", [1, 2, 3, 4, 5], is_input=True)
+        fa.append(1.3333)
+        self.assertEqual(fa.pytype, float)
+        self.assertEqual(fa.dtype, np.double)
+
+        fa = FieldArray("y", [1.1, 2.2, 3.3, 4.4, 5.5], is_input=False)
+        fa.append(10)
+        self.assertEqual(fa.pytype, float)
+        self.assertEqual(fa.dtype, np.double)
+
+        fa = FieldArray("y", ["a", "b", "c", "d"], is_input=False)
+        fa.append("e")
+        self.assertEqual(fa.dtype, np.str)
+        self.assertEqual(fa.pytype, str)