china-the former agency refuses to release letter of recomme...

Good Luck! I have lived there and the business practices do not exist. It a money maker for them only. Maybe consider a colleague from there to provide you a letter.

it seems that once you have worked in china, it has to be the last employer that issues the official letter, to match with the old contract and other documents thanks for the kind support Ms Foreman

I would stay calm and try to have a sit down conversation with the boss. Ask him what the real reason is for not providing the letter and if there's anything you can do to change his mind. If you can't, just move on. They're not always strictly required by the next employer. I also think it is still a good idea to ask your (previous) co-workers and managers for a character reference. Simply explain to the next employer that your boss wouldn't release a letter of recommendation because you were not interested in resigning...