Abstract: The water of the once-through steam generator secondary side goes through complicated phase change processes, and the flow types and heat transfer cases are much more complex than that of the natural circulation steam generator used in pressurized water reactor. The straight pipe type once-through steam generator experiment equipment of B&W Company was studied in the paper. The best estimated code RELAP5/MOD3.4 was chosen, and the influences of the node number, calculation time step, arithmetic, and the analysis method were studied in detail. The uncertainty factors that should be careful attention during the research on the great phase change were described. The results show that the number of the control volume increases; the calculation time step decreases; the two-phase model is adopt properly based on problem type; the using condition of the different arithmetics is ascertain; the multi-channel analysis method is selected accordingly.