Abstract: If loss of main feed-water occurs in a station blackout accident for AP1000 plant, the pressurizer will overfill and the coolant will be discharged through pressurizer safety valves. It results in a loss of coolant accident, RCS inventory will decrease, and the risk of reactor core uncovering increases. Because of the coolant discharging, the atmosphere radiation level in the containment may be raised, while the possibility of radioactive release to the environment increases. In order to prevent pressurizer overfilling, an effective strategy to avoid and mitigate pressurizer overfilling was provided. The results show that increasing heat transfer areas of PRHRS heat exchanger can prevent pressurizer overfilling; reasonable decreasing of IRWST back pressure can enhance margins of pressurizer overfilling, and mitigate pressurizer overfilling phenomena; increasing pressurizer volumes can also avoid pressurizer overfilling. The conclusions have reference value in helping design and safety analysis of AP1000 plant.