Abstract: The effects of different boundary conditions and physical models on two-phase flow instability boundary were studied in this paper. In order to verify the code and model, the RELAP5 code was used to simulate two-phase flow instability experiments in once-through evaporation tube. Then the influences of constant flow rate and constant pressure drop boundary conditions, the number of parallel tubes, the axial power distribution patterns and the heat capacity of the heat transfer tubes on the instability boundary were analyzed. The results show that the instability boundary differences among single tube, two tubes and multiple tube models are less than 5% under constant pressure drop boundary conditions. Under constant flow rate boundary conditions, the instability boundary of multiple tube models differs from that of two tubes model by less than 5%, while the instability boundary of single tube model differs from that of two tubes models by more than 100%. When the numbers of parallel tubes in the models are the same, the stability of the model with the constant flow rate boundary conditions is better than that with the constant pressure drop boundary conditions. The stability of the system with increasing power distribution along the flow direction (axial direction) is better than that with uniform power distribution, and the stability of the system with uniform power distribution along the flow direction is better than that with decreasing power distribution. When the wall thickness varies from 0 to 20 mm, the wall heat capacity has little effect on the instability boundary.