Abstract: Based on the best estimate program RELAP5/MOD3.3, the reactor coolant system and the passive core cooling system of AP1000 nuclear power plant were modeled and analyzed. Some transient parameters of automatic depressurization system (ADS) inadvertent actuation accident were obtained, including system pressure, break discharge, system inventory, etc. The result was compared with the result generated by NOTRUMP code from Westinghouse, and the difference between them was discussed. It is proved that the passive safety system of APl000 nuclear power plant can effectively lower the temperature and reduce the pressure of the primary loop, and also can prevent core overheating. The safety of AP1000 nuclear power plant during the accident caused by inadvertent actuation of ADS was verified.