Call to search API with Application-only authentication always gets 403 error



What I want to do is very easy, search twitter with a keyword, then display the result. It was super easy with API v1.0, now it becomes not that easy.
Because I don’t need any user information, I think Application-only authentication should work for me.
I’ve already successfully obtain the token from server, but when I called the search API with the token, I always got 403 error.
Totally have no idea about the reason. Any suggestions are very welcome.