Chinese people can re-apply passport in Cambodia or not?(his passport will expire soon)

Yes, you can try to renew your Chinese passport at the China embassy or consulate in Cambodia. You can check their official website for detailed information.

How to process it and how long ?

Hi, first, make an online appointment for the application.
Prepare your passport and its copy, a completed application form, and a passport-style photo, residence permit in there and the copy, and then submit the application to the embassy or the consulate where you resides. It would take 15 working days for the processing.