Probability Safety Assessment (PSA) has been widely used in the design of nuclear power plant in China, but most of the applications are based on internal events, and there are few applications based on internal and external hazards. This article presents two specific examples of recent applications for internal fire and seismic in the design of nuclear power plants. These two examples illustrate the importance of hazard PSA to improve the level of defense-in-depth (DID) and the safety performance. In view of the fact that there are no conditions for establishing a detailed hazard PSA model in the early stage of design, and it is often difficult to find risks in the later design phase and then adjust the design. This paper proposes a method to identify high-risk hazard sequences in the early stage of design, which can identify potential risks related to hazards earlier, so that the PSA method can intervene in advance to support the design work related to layout and hazard protection.