Abstract: Probabilistic safety assessment (PSA) methodology was used to identify the availability of service water system (SWS) and the corresponding plant risk for Sanmen and Haiyang nuclear power plants, based on the actual design of SWS of the two plants, as well as the system operation requirements, especially the effect of the system layout. According to the results of the risk analysis, the reasonable modification suggestions on design deficiencies were provided for Sanmen Nuclear Power Plant, and the modification was unnecessary for Haiyang Nuclear Power Plant. It is shown that PSA functions as the support and instruction to the design.