I have a question:How to improve my vocabulary??

Do more reading. It works for me.

It is an interesting question. - I believe it might be useful to focus on your areas of interest (in writing). - Vocabularies are many times different for different "areas" (genre) (of interest). - What might be useful in writing "common stories" (or articles) would be less useful in writing about computers or astrophysics. - If you are going to write about computers, then read all you can that is written about computers and accumulate your vocabulary from those sources (and, meanwhile, learn what is of interest at the moment). - Also, write, write, write! - (That should be no "news" for you.) - Best of Fortune to you. - Gary.

My suggestion would be to use a vocabulary text book AND to watch or listen to as much English language media as possible, but ALSO do the following. Every day, in a notebook which you can carry with you, jot down a list of five or so (but no more than ten or twelve) words or phrases which you have come across and want to learn and, in spare moments during that day, choose a word, speak it (either out loud or under your breath), if possible write it and try to think of a sentence in which you might use it.