Abstract: Based on the power conversion system of nuclear and conventional islands of Daya Bay Power Station,this paper models the thermal-hydraulic systems of primary and secondary loops for PWR by using the PWR best-estimate program-RELAP5.To simulate the full-scope power conversion system,not only the traditional basic system models of nuclear island,but also the major system models of conventional island are all considered and modeled.A comparison between the calculated results and the actual data of reactor demonstrates a fine match for Daya Bay Nuclear Power Station,and manifests the feasibility in simulating full-scope power conversion system of PWR by RELAP5 at the same time.