Abstract: The vertical U-tube natural-circuit steam generator of AP1000 was modeled and simulated. According to the characteristics of thermal-hydraulic, the characteristics of fluid flow, the operating state, and the modeling and simulation principle of RELAP5, the steam generator was divided into several nodes, so as to determine the type of each component and the connection between the various components. Then the initial parameters of the component were entered to the RELAP5 input card, and the goal of build a model for the steam generator was achieved. Finally, the level change curves of steam generator were obtained under different parameter changes. The results show that the simulation results of RELAP5 are consistent with the actual situation.