diff --git a/README.md b/README.md
index d975d3730569dd503b781531a1f2de78de018056..b6ae2beed999d146c64ffc9ee495373d9b77a175 100644
--- a/README.md
+++ b/README.md
@@ -11,6 +11,24 @@ This project is used to develop the next version deep learning freamwork for mob
 
 [Used model in development](https://mms-mis.cdn.bcebos.com/paddle-mobile/models.zip)
 
+## cross-compilation to android
+
+* NDK is required
+* ANDROID_NDK environment variable is required
+
+```bash 
+sh build.sh android
+```
+
+## build for x86
+paddle-mobile is to run on arm platform. x86 only used to test not arm assembly code. So do not recommend compiling x86.
+
+Now only support osx.
+
+```
+sh build.sh mac
+```
+
 ## Old Version of Mobile-Deep-Learning
 The old version of MDL was I moved to here [Mobile-Deep-Learning](https://github.com/allonli/mobile-deep-learning) 
 
diff --git a/build.sh b/build.sh
index 51ee36401ac8a92ff909884075a7ed862787cb97..dc31f3b47f47975309e46c063bee142d1cc1a14f 100755
--- a/build.sh
+++ b/build.sh
@@ -1,39 +1,7 @@
-#!/bin/bash
-
-build_for_linux() {
-	if [ ! `which brew` ]; then
-        echo "building failed! homebrew not found, please install homebrew."
-        return
-    fi
-    if [ ! `which cmake` ]; then
-        echo "installing cmake."
-        brew install cmake
-        if [ ! $? ]; then
-            echo "cmake install failed."
-            return
-        fi
-    fi
-    PLATFORM="x86"
-    MODE="Release"
-    CXX_FLAGS="-std=c++11 -O3 -s"
-    BUILD_DIR=build/release/"${PLATFORM}"
-    mkdir -p ${BUILD_DIR}/build
-
-    mkdir -p ${BUILD_DIR}/test
-    cp -r test/models ${BUILD_DIR}/test/models
-
-    cmake . \
-        -B"${BUILD_DIR}" \
-    	-DCMAKE_BUILD_TYPE="${MODE}" \
-    	-DCMAKE_CXX_FLAGS="${CXX_FLAGS}" \
-    	-DIS_MAC=true
-
-    cd ${BUILD_DIR}
-    make -j 8
-}
+#!/usr/bin/env bash
 
 build_for_mac() {
-	if [ ! `which brew` ]; then
+    if [ ! `which brew` ]; then
         echo "building failed! homebrew not found, please install homebrew."
         return
     fi
@@ -65,8 +33,8 @@ build_for_mac() {
 }
 
 build_for_android() {
-    if [ -z "${NDK_ROOT}" ]; then
-        echo "NDK_ROOT not found!"
+    if [ -z "${ANDROID_NDK}" ]; then
+        echo "ANDROID_NDK not found!"
         exit -1
     fi
 
@@ -148,4 +116,4 @@ else
 	else
 		build_error
 	fi
-fi
\ No newline at end of file
+fi