How to extend a Z visa?

After obtaining a Z visa from a Chinese visa office, a foreign expert should pay close attention to the expiry date in China. If the person needs to extend or replace a Z visa, he or she needs to submit an application three days before the expiry date to a visa office, which can be done at any local public security bureau’s exit-entry administration.

Because no application for a visa with a later expiry date than that of the applicant’s passport can be accepted, the applicant needs to pay special attention to the passport expiry date.

In leaving China at the time the visa expires, the foreign expert does not need to cancel a single-entry visa.

(The info from State Administration of Foreign Experts Affairs China)