I’m going to china on 30 days double entry visa.. how can I extend it in china after the completion

To extend the visa in mainland China, first, you may need to provide your passport, the completed application form, a recently-taken bareheaded color photo, registration form of temporary residence, as well as other requirements, and then go to the exit and entry administration of the local PSB to apply for the extension. You can also contact PSB to inquire the detailed information before you do the extension.