New Zealand Embassy Beijing, China
It is usually straightforward to change employers at the end of a contract. To do so requires a new contract, a letter of reference from the original employer and the approval of the PSB.
Changing one's employer part way through a contract is more difficult. This requires release consent from the original sponsor for the remaining contract period and again approval by the PSB. It may also mean leaving China to arrange a new work visa.
Any questions on this procedure should be directed to your nearest PSB office or Chinese embassy or consulate-general.
If you do resign to take up new employment without a letter of release from your previous employer, then you must leave the country within nine (9) days of your resignation. A new visa will not be issued until the expiration of your previous contract. However, after exiting the country you can return to China as a tourist while awaiting the expiration of the old contract - but you cannot work.