Abstract: In the Probabilistic Safety Assessment (PSA), Fault Tree (FT) is the approach commonly used for system reliability analysis. And it is the important feature of passive system and the basic difference from the active system that nuclear plant is able to be driven to safe state or shutdown by inherent safety characters of the reactor and physical principles, and is independent of human interfere or the operations of outside equipments, when the reactor is in the abnormal condition. Therefore, the passive system is widely used in new generation Nuclear Power Plant(NPP), such as AP1000 NPPs and high-temperature gas-cooled reactors to improve the safety. At the same time, since the passive system operation is depending on natural force, both the driven force and resistance are influenced by many uncertain factors, physical process failure become one of the significant causes for the system failure, which need to be considered in the system reliability analysis. FT method is not power enough to deal with this condition. For the passive system which is operating based on natural circulation, temperature of heat source and sink are important influence factors on system operation. Monte Carlo (MC) simulation is used to analyze the influence of heat source and sink temperature on system operation. From the result, it could be seen that the less the number of operation train is, the more prominent influence is. And if the safety margin is big enough, the system might not fail even when the system configuration does not satisfy design requirement, that is, the system failure is the integrate result of physical process failure and device failure which are mutually conditional.