Abstract: Loss of Coolant Accident（LOCA） with medium break and small break in the pipes directly joined to the primary coolant are analyzed in the case of MNPP is in full power operation condition.The requirements for safety functions in LOCA,the effects of the responses of mitigation systems and intervening behaviors of operators on core damage after initiating events are studied.During modeling accident sequences,the large event trees/small fault trees approach is adopted based on the simplification of support systems and front-line systems.By quantitative analysis,the dominative sequences and events led to core damage under initiating events are founded.Effective suggestions on system improvement and discussion on the application of large event tree/small event tree in MNPP PSA are also presented.