Abstract: Under the natural convection condition, a heat transfer model of steam/air condensation on the vertical tube external surface is built. In the modeling process, considering that some steam which liquefies when not reaching the gas-liquid interface can enhance heat transfer, the natural convective heat transfer coefficient is corrected; when calculating the condensation heat conductivity, the integration algorithm considers changes of the gas mixture density in the diffusion boundary layer. The results show that the deviation between predicted values of the model and Dehbi’s experiment results is less than 15%, and the maximum deviation against the Anderson’s experimental results is 16.8%.Compared the present model with the existing models, it is found that the new model has higher precision than the other two old models.