Abstract: Molten pool cooling by cavity flooding strategy plays an important role to mitigate severe accident consequences and decrease the containment failure probability. SCDAP/RELAP5 code was used in this paper. Firstly, the code calculation results were compared with the Korea experiment of APR1400, and the capability for natural circulation cooling analysis with cavity flooding was validated. Then, the model of a passive PWR was built for analyzing molten pool cooling coupled with cavity flooding strategy. For severe accident induced by large break loss of coolant accident (LBLOCA) at cold leg, molten pool cooling characteristics during core damage progression were analyzed. After cavity flooding strategy was implemented, heat removal capability by the natural circulation was also analyzed. The results indicate that molten pool in the lower head can be effectively cooled by cavity flooding after core exit temperature exceeds 923 K during LBLOCA, and the integrity of RPV can be maintained.