Abstract: The two-phase flow field equation can be classified as three equation, four equation, five equation and six equation models respectively, according to the differences of the flow and heat transfer characteristic. Based on the field equations and the related flow pressure drop model, heat transfer model, two-phase interaction model, fluid state parameters and thermal properties of structure material, the design and research of the steam generation equipment and system can be properly analyzed. The characteristics and limitations of the two-phase flow field equations with different numbers were analyzed. Based on the once-through steam generator experiment data, four different two-phase flow field equation models in the best-estimate codes were adopted to calculate and analyze the flow and heat transfer characteristics, in which the key issue was emphasized on comparisons of pressure and heat transfer characteristics of the forced convective flow in the process of heating the subcooled water to the superheated steam. The differences of the relevant flow field models in phase change process were listed. The prediction capabilities of the RELAP5 code and RETRAN-3D code for once-through steam generator were verified. The results show that the six equation model of the RELAP5 code is better than other models in simulating once-through steam generator flow and heat transfer characteristics.