Abstract: Aiming at generating electric energy and producing fresh water, the marine nuclear power platform (MNPP) provides support for the national energy strategy. Considering the coupling effect between steam condensation and nature convection, a three-dimensional condensation heat transfer/one-dimensional natural circulation coupling numerical method was developed to analyze the passive compartment cooling system (PCCS) operating performance. Using Fluent and UDF codes, models of PCCS heat exchanging process between the two sides of heat transfer tubes were established. Then the major thermal-hydraulic parameter responses of PCCS were obtained. The results show that when PCCS working, the steam can condense effectively, and both the temperature and pressure in reactor compartment decrease. The analysis method can provide guidance for MNPP PCCS design.