This thesis investigates the heat transfer performance of the passive safety cooling system (PCCS) in nuclear power plants, with a focus on analyzing the influence of liquid filling rate, design parameters, and natural circulation on system efficiency. As an important design feature of the third-generation advanced nuclear reactors, PCCS relies on the principle of natural circulation to achieve long-term and reliable heat dissipation without external power or human intervention, significantly enhancing the safety of nuclear power plants. Through the Relap system analysis program, this paper explores the impact of liquid filling rate on the thermal conduction efficiency of the system, indicating that the optimization of liquid filling rate at different power levels has a significant effect on the phase change heat transfer of heat pipes and the driving force of natural circulation. Additionally, the study shows that a reasonable height difference design can enhance the natural circulation effect of the system, thereby improving the heat exchange efficiency. This research provides a theoretical basis for the optimization design of PCCS, particularly in terms of the key influences of liquid filling rate and design parameters, offering important references for future nuclear power safety design and improvement.