Abstract: Due to the requirements of safety level of nuclear power plant are gradually improving, more and more passive systems are used in the advanced type reactor. However, the reliability evaluation work for these passive systems is still in the primary stage. In this paper, based on the reliability evaluation process of passive system, the physical process of passive system was simulated by RELAP5 thermal-hydraulics program and the reliability of AP1000 external reactor vessel cooling (ERVC) system was evaluated. After the large number of calculations, the cumulative density distribution curve of the temperature in lower head of the reactor pressure vessel and other parameters were obtained. Combined with specific success criteria, the reliability of AP1000 ERVC system was got. This reliability evaluation result of ERVC system can be used in the PSA model to better guide the design of nuclear power plants and improve the safety of nuclear power plant.