Abstract: The passive residual heat removal system（PRHRS） can work for a long period of time to achieve its function by its natural cycle characteristics, ensuring the reactor safety. To meet this requirement, A type of passive residual heat removal system（PRHRS） is designed, the emergency cooling water tank is worked as an intermediate buffer device, ensuring the rapid cooling in the early stage of the accident and the PRHRS could work reliably for a long period of time. Based on the natural circulation characteristics, design calculation gave a steady-state solution. The RELAP5 code was used to analyze the operation characteristics of the PRHRS and the transient characteristics of the primary loop system affected by the PRHRS. The results show that: through rational design, the residual heat can be effectively removed by natural circulation in the PRHRS, guaranteeing the security.