Since Android Studio uses the new Gradle-based build system, you should be putting assets/ inside of the source sets (e.g., src/main/assets/), if I understand correctly.

In Android Studio, right-click on the  folder and navigate to the Assets Folder.

On the next screen just click Finish.

It will create the assets folder in the main target source set.

法三：

Looking inside the .iml file of your project you will see the following line:

 <option name="ASSETS_FOLDER_RELATIVE_PATH" value="/src/main/assets" />

Therefore the "assets" folder is already declared for Gradle but you need to create it under src/main/ (I'm using AS 0.4.2)