Abstract: Passive residual heat removal (PRHR) system of AP1000 nuclear power plant is one of the important safety systems. The reliability of PRHR system was estimated by using fault tree analysis (FTA), and the potential failure mechanism of PRHR system was got. The software of Risk Spectrum was used to make quantitative analysis of PRHR system, and the probability for failure of the system was about 9.215×10^-5. The final result shows that the motor operated isolation valve of the PRHR heat exchanger inlet failure to open is the most important factor leading to the system failure. Based on the results, two methods were put forward to optimize the system:1) The same PRHR system was added in another loop; 2) The PRHR heat exchanger was added in original system. By the fault tree analysis and calculation, and comparing the two methods separately from the system reliability, complexity, economic and other aspects, method 2 is more feasibility, and this method is proposed to be used to optimize the system.