Abstract: The primary loop of CPR1000 nuclear power plant was modeled using RELAP5/MOD3.4 code, and the transient thermal hydraulic characteristics were analyzed under the condition of station blackout accident (SBO). The calculation results by RELAP5 code were compared with those of THEMIS code, and the results by RELAP5 code were consistent with those of THEMIS code. The results show that the RELAP5 model can accurately simulate the transient thermal hydraulic characteristics of CPR1000 under the condition of SBO.