The ram air system of civil aircraft brings the environmental air through the inlet apparatus on the aircraft skin and applies the cool air with applicable pressure and air flow rate for air condition system, supplemental cooling system and auxiliary propulsion system, which affects the economic and comfortable performance of the aircraft. The ram air system consists of the inlet apparatus, the outlet apparatus, the heat exchangers and the fan and the ducts. The complex flow characteristics determine the cool air flow distribution for each user. In order to obtain the proper air flow distribution of ram air, the three-dimension simulation method needs to be established. Because the aircraft outside environment and the inside components were affect the performance of the ram air performance, the numerical models for outside part and inside part were established. CFD method was used to calculate the flow trend in the ram air channel, and the flow distribution results of the ram air system and static pressure distribution in the channel were obtained, which can provide an example for the global design of ram air system.