A recommendation letter from your direct boss including what you have done on the position and his evaluation of you would be good. and you can also ask the company to issue an official certificate of work experience. The former is acceptable better if your boss includes his address and phone number on the letter so the immigration officer can contact and verify. the latter is more chinese official way. I did both of them. and if you have evidences of awards, prize, published reports, papers etc.they are very good to confirm your experience and achievement. remember, all these should match your resume, never conflict with each other. Good Luck!
