Abstract: The scale-down separate and integral effect test facilities are of vital significance not only for the research and development of large-scale advanced pressurized water reactor nuclear power plants, but also for validation and verification of the related analysis codes. The passive containment cooling system (PCS) air flow path system plays an important role in containment passive cooling. Based on the natural circulation model, the scaling and design methods for both the separate and integral effect test facilities were proposed. Under given heat flux from the inner side of the shell, simulation of an ideal scaled facility (ISF) design of a 2/5 length scaled separate effect test facility for CAP1400 PCS was preliminarily performed by the PCCSAP-3D code. The simulation results show that the design method and the modeling of natural circulation on a reduced height test facility are feasible.