This paper builds simplified physical model, according to the outside structures of the passive containment cooling system, to study the accident residual heat removal by air cooling when nuclear power plant accident occurs. Numerical simulation is used to set up the air circulation outside of the steel containment and provides the heat transfer process, including air temperature, velocity, and pressure for three different sections. By analysis, it also obtains and discuss the heat flux transferred out from the steel containment and the heat flux from the baffle plate to the decline section. Finally,it obtains the main heat transfer models for the decline section, the ascent section and the dome section. Results provide supports to enhance core residual heat removal.