Abstract: Based on the PWR best-estimate program RELAP5/MOD3.4, the reactor coolant system and the passive core cooling system of AP1000 were modeled and analyzed. Some key transient parameters were obtained, including RCS pressure, break discharge, cladding temperature, etc. The result is in close agreement with the result generated by NOTRUMP code from Westinghouse (USA). The calculation results show that safeguard system of AP1000 can effectively lower the temperature and reduce the pressure of the first loop, also can prevent core overheating. The safety of AP1000 during a small-break loss-of-coolant accident is verified.