For China domestic flights: you can select the seat when changing the boarding pass at the airport; you can also do that by mobile check-in, online check-in or self-service check-in kiosk.

For international flights, different airlines have varied policies. Some carriers provide seats-selecting by online check-in; some airlines require you inform the airline staff by phone; while some airways only offer seats-selecting service for their affiliate, and sometimes you need to apply for a preferred seat when changing the boarding pass at the airport.