Abstract: A simulation model by utilizing RELAP5 was established for the external reactor vessel cooling (ERVC) system to investigate the effects of various thermal-hydraulic parameters such as containment pressure, subcooling, heating power and flooding water levels under the condition of cavity flooding balance on two-phase natural circulation flow capability of the system. The critical subcooling and instability boundary were found at given conditions. The results show that the thermal margin of ERVC system for AP1000 design is so large that the corium could be cooled down through lower head wall only by natural circulation. Flow rate of natural circulation increases as the containment pressure increases, the cavity water subcooling decreases, the heating power increases, and the flooding water level increases. Besides, pressure is not sensitive to critical subcooling when the heating power is low enough. When the subcooling is below the critical value, intensive flow oscillation and back flow occur, and the steady flow can not be established when the cavity water level is below 5.5 m.