Abstract: The once-through steam generator operated by B&W was used as a prototype and the single-phase liquid convective heat transfer region, nucleate boiling region, post-dryout region and superheating steam region (full range) in secondary side of an once-through steam generator were predicted by using two-fluid three-flow-field mathematical model. The results show that the model used in this paper can accurately predict the full range flow and heat transfer characteristics and the numerical results are in good agreement with operational data. Post-dryout steam quality is lower than thermal equilibrium steam quality and there exists thermal non-equilibrium in post-dryout region of the running steam generator combining with the wall temperature distribution. With the development of flow and heat transfer, surface heat transfer coefficient increases rapidly, the occurrence of dryout leads to a sharp decline in heat transfer performance and then surface heat transfer coefficient rises slowly in post-dryout region and superheating steam region.