Abstract: The reactor primary cycle and passive core cool system were simulated by use of RELAP5/MOD3.3 code, then we can conclude transient state operating characteristics of AP1000 passive residual heat removal system (PRHRS) under loss of power accident. The results show that: Condition 1, PRHRS starts on low steam generator water level coincident with low start up feed water flow, opening of pressurizer safety valves will lead coolant reverse flow in the PRHRS, and arouse coolant boiling in the core and excessive temperature differentials between inlet outlet of pressure vessel; condition 2, PRHRS starts directly on power loss signal, safety valves will not open before it starts, so PRHRS is capable of operating effectively.