The once-through steam generator designed by Babcock & Wilcox is used as the prototype. The two-fluid three-flow- field mathematical model is introduced and the wall heat transfer conditions are re-partitioned in present work. The model is used to calculate the flow and heat transfer involving the single-phase subcooled water region, nucleate boiling region, dryout phenomenon, post-dryout region, and single-phase superheated steam region, i.e., the "full range" of the process. The results show that the proposed model can accurately predict fullrange thermal-hydraulic characteristics and post-dryout deviation from thermodynamic equilibrium. The heat transfer type in the nucleate boiling region was mainly evaporation heat transfer and accompanied with liquid heat convection and quenching heat transfer. Furthermore, the heat transfer coefficient decreased and the wall temperature rose sharply at the dryout position with the heat transfer type turning into steam heat convection in the post-dryout region. Investigation of the level of deviation from thermodynamic equilibrium in the postdryout region was conducted by defining a new parameter post-dl, with the results showing that the level of deviation decreased gradually with the development of flow and heat transfer in the post-dryout region, and the average deviation was about 42% when the once-through steam generator ran at full load.