Abstract: In order to improve the reliability of Residual Heat Removal System(RHRS) in Pressurized Water Reactor nuclear power plant and provide guide and reference for regular overhaul and maintenance of RHRS, the reliability modeling of RHRS was conducted by using fault tree, and the importance index of system components was evaluated. Based on the evaluation results of importance index, an improved scheme of cold standby system with common spare parts is proposed. Dynamic fault tree modeling is adopted and Markov model is used for quantitative analysis. The results show that the reliability of the optimized RHRS is significantly improved.