Abstract: The AP1000 is the typical “third generation” passive nuclear power plant in the world at present. The primary system of AP1000 nuclear power plant was modeled using RELAP5/MOD3.3 code, and the transient thermal-hydraulic characteristics and the response characteristics of passive system were analyzed under the accident sequence of main feedwater line rupture accident. The results show that during the accident, the primary loop pressure, secondary loop pressure and primary loop temperature are fluctuant. The RCS pressure maximum value is 17.13 MPa, less than 91% of the design pressure. The pressure of the main steam system is also less than 91% of the designed value, which satisfies acceptance criteria.