A typical 1000MW pressurized-water reactor (PWR) unit model of China’s living nuclear power plant (NPP) units is built based on MAAP4[1] in this paper. Different severe accidents cases caused by different LOCA area on hot leg of primary loop are studied. And different mitigation measures are focused to evaluated their effectiveness. The study indicates that during the accident, the larger broken area LOCA case caused the more severe rector core damaged. However, it is important to inject water into the reactor core in good time. And ...