Abstract: According to the thermal-hydraulic characteristics of once-through steam generator (OTSG) of sodium-cooled fast reactors, the OTSG transient analysis code OTAC was established based on the fixed and movable boundary models. The United States Energy Technology Engineering Center OTSG shutdown experiment was simulated by OTAC, and the calculated and experimental results were compared and analyzed. The calculated values of the two models are in good agreement with the experimental values. The step results can be avoided only in enough grids for the fixed boundary model. However, the numerical stability and the continuous calculation results can be improved based on less number of the grid for the movable boundary model.