Abstract: Sodium-water once-through steam generator is one of the most important components for removing heat from the main heat transfer system of fast reactor, and its structure and internal heat transfer phenomenon are very complex. Medium and pressure inside and outside of the steam generator heat transfer tubes are different. Water/steam with high temperature and pressure is on the tube side and where the complicated heat and mass transfer of two-phase flow occurs. Liquid sodium is on the shell side, and there is a great change in temperature along the height of heat transfer tubes. In this paper, the thermal hydraulic characteristics of sodium-water once-through steam generator seven-tube prototype were studied using CFD software and experiment method. The CFD analysis results and experimental results are in good agreement, and the reliability of the mathematical models and numerical methods used in CFD analysis is verified. The results show that there is sufficient heat transfer area to meet the design requirements for the steam generator, the critical mass vapor fraction is about 0.42, the critical heat flux is about 451.98 kW/m², thus the location of dry-out is determined.