Transit visa at Beiging Capital Airpirt (PEK)

I am a Sri Lankan passport holder and planning to travel Ulaambaata (Mongolia) via Beiging Capital Airpirt (PEK) for in bound and outbound transfer. Travel between Beijing and Ulaambaata (Mongolia) using Air China and from/to Colombo to Beijing using Sri Lankan Airline.

Transit time not exceeding 12 hours. Do I need transit G (for China).