I want to cooprate with my chinese friend in a restaurant what visa shall I apply for?

Hi there! I think you can apply for a Z type or M type according to the stay length you expect. If you want to stay for a short term, choose the M type. If you want to stay for a longer time, choose the Z type. You need to get an invitation letter from the restaurant to apply for the M type. To apply for the Z type, you need to get a work permit as well as an invitation letter from a duly authorized unit in PRC.