Abstract: To study the transient thermal-hydraulic characteristics of the lead-bismuth fast reactor, the system thermal-hydraulic analysis code RELAP5 was modified with lead-bismuth eutectic (LBE) properties and heat transfer models. Verification using the NACIE-UP facility and the CIRCE-ICE facility was completed. The calculation results show that the steady-state relative error and transient relative error for the NACIE-UP facility are within 2% and 5% respectively. And compared with other system thermal-hydraulic analysis codes, such as CATHARE, ATHLET, RELAP5-3D and RELAP5/MOD3.3 (modified), the relative deviation is within 10%. The steady-state relative error and transient relative error for the CIRCE-ICE facility are within 2% and 10% respectively. This code can meet the accuracy requirements of thermal-hydraulic analysis code of reactor system, and can be used as an effective tool for safety analysis of lead bismuth fast reactor.