Abstract: The passive residual heat removal system (PRHRS) of a small lead-bismuth fast reactor is mainly used to deal with station black-out (SBO) accident. However, it is uncertain whether the PRHRS can effectively remove core decay heat to ensure core safety. Therefore, numerical analysis was carried out to evaluate the residual heat removal capability of PRHRS. The RELAP5 4.0 code was used to simulate thermohydraulic characteristics of the small lead-bismuth fast reactor under the SBO accident. The steady-state calculation was carried out first, and then the steady-state results were taken as the initial values for transient calculation. The results show that in the whole SBO accident, the peak cladding temperature is 820 K and the maximum wall temperatures of main vessel and guard vessel are 792 K and 769 K respectively, which are within the safety limit, indicating that the PRHRS can effectively deal SBO accident. This study can lay a technical foundation for the PRHRS engineering design of the small lead-bismuth fast reactors.