Abstract: After the Fukushima accident, China has paid special attention to hazards beyond design basis. Nuclear power plant should have sufficient margin to deal with hazards beyond design basis. Among them, external flooding is one of the external hazards with important risks.At present, the risk assessment of external flooding beyond design basis is still under study,and there is no mature and unified risk assessment method at domestic and abroad. Based on domestic and foreign studies and engineering practices, this paper develops the methodology of external flooding probability safety analysis（PSA）, and applies it to the risk assessment of external flooding for a certain nuclear power plant, identifies the weak items and optimizes the protection design against external flooding.