找了半天没找到，还是用google好啊！修改如下，打开如下的build.gradle，修改compileSdkVersion 为你下载的源码版本号。
Open your sdk manager from toolbar menus and download the "Sources for Android SDK" for the API level defined in your build.gradle file like
compileSdkVersion 19 // compile with API level 19
Nothing else needs to be done.
Note : Sources for SDK is available only for API levels 14 and above.
参考了stackoverflow。
原文：http://www.cnblogs.com/xdao/p/android-studio-how-to-attach-android-sdk-sources.html