Abstract: Simulation calculations of AP1000 nuclear power plant consisting of the primary and secondary loop systems, and the passive safety systems with RELAP5/MOD3.3 code were performed under the assumption of Fukushima nuclear accident. Some key transient parameters including RCS pressure, core coolant temperature, and passive safety system flow were obtained. The analysis results show that the AP1000 reactor can reach hot shutdown status with the passive residual heat removal system (PRHRS) available; however the reactor will undergo severe accident in five hours with the assumption of double-ended rupture of one PRHRS tube.