diff --git a/paddle/fluid/operators/reader/create_double_buffer_reader_op.cc b/paddle/fluid/operators/reader/create_double_buffer_reader_op.cc
index 5f35b9b3eac1d9aab8662833c6e39d12f11a0087..5f734489a81764875988f440696682570ff4d1d7 100644
--- a/paddle/fluid/operators/reader/create_double_buffer_reader_op.cc
+++ b/paddle/fluid/operators/reader/create_double_buffer_reader_op.cc
@@ -23,13 +23,13 @@ namespace reader {
 
 // 'Double buffer' means we shall maintain two batches of input data at the same
 // time. So the kCacheSize shoul be at least 2.
-static constexpr size_t kCacheSize = 3;
+static constexpr size_t kCacheSize = 5;
 // There will be two bacthes out of the channel during training:
 // 1. the one waiting to be sent to the channel
 // 2. the one just be received from the channel, which is also being used by
 // subsequent operators.
 // So the channel size should be kChacheSize - 2
-static constexpr size_t kChannelSize = 1;  // kCacheSize - 2
+static constexpr size_t kChannelSize = 3;  // kCacheSize - 2
 
 class DoubleBufferReader : public framework::DecoratedReader {
  public:
diff --git a/python/paddle/fluid/layers/io.py b/python/paddle/fluid/layers/io.py
index 8d153b75cd49953770cfa89348914a375be82a82..9b4be058003f853a006a8da77a0016a6e733854c 100644
--- a/python/paddle/fluid/layers/io.py
+++ b/python/paddle/fluid/layers/io.py
@@ -110,7 +110,7 @@ class BlockGuardServ(BlockGuard):
 class ListenAndServ(object):
     """
     **ListenAndServ Layer**
-    
+
     ListenAndServ is used to create a rpc server bind and listen
     on specific TCP port, this server will run the sub-block when
     received variables from clients.
@@ -212,7 +212,7 @@ def Send(endpoints, send_vars, sync=True):
                    of send_vars to send
         send_vars (list): variables to send to server
         sync (bool): whether to wait the request finish
-    
+
     """
     assert (type(send_vars) == list)
 
@@ -469,10 +469,13 @@ def open_files(filenames,
        lod_levels(list): List of ints which declaring data lod_level.
        dtypes(list): List of strs which declaring data type.
        thread_num(int): The maximal concurrent prefetch thread number.
-       buffer_size(int): The size of prefetch buffer.
+       buffer_size(int|None): The size of prefetch buffer. If it is setted None, 
+            buffer size will be thread_num * 3.
+            Default: None
        pass_num(int): Number of passes to run.
        for_parallel(Bool): Set it as True if you are going to run 
             subsequent operators in parallel.
+            Default: True
 
     Returns:
        Variable: A Reader Variable via which we can get file data.
@@ -492,7 +495,7 @@ def open_files(filenames,
          image, label = fluid.layers.io.read_file(reader)
     """
     if buffer_size is None:
-        buffer_size = thread_num
+        buffer_size = thread_num * 3
     if isinstance(filenames, basestring):
         filenames = [filenames]
     dtypes = [convert_np_dtype_to_dtype_(dt) for dt in dtypes]