Abstract: Based on the conservation equations of fluid mass, momentum and energy, a thermal-hydraulic model for helical-coil tube type once-through steam generator (HCSG) was developed by movable boundary method. The heat transfer control volume was divided into three segments: economizer, evaporator and superheater regions, and the mist evaporation region was taken into account. The thermal-hydraulic model was evaluated against the data of the HCSG of the International Reactor Innovative and Secure (IRIS). The steady state results of the new model are in good agreement with the design parameters of IRIS, and the dynamic simulation results are suitable for the thermal-hydraulic analysis of HCSG.