Yangchenyun, whenever I hear remember or memorize in a programming context, my alarm bells start ringing.
You need to understand what a certain part of the API is doing and what concepts the API is based on. Then the fact that the API is able to do a certain thing will stay in your head automatically, even if you don't have all the details present in your head at any time.
After that Mad Keith's and Geerten's answers will kick in.
Just keep programming in your language of choice and your API knowledge will increase automatically.