The down-flow unit tube is a key component of air cooled heat exchanger (ACHE) in power plant. The study on the heat and mass transfer mechanism inside the unit tube is very important for heat-transfer efficiency optimization and safe operation of ACHE. In this paper, considering the geometry characteristics of tube, a 3D mathematical model was constructed to simulate the condensation of steam on the internal wall of unit tube. The temperature of tube wall was assumed to be unknown and an iterative algorithm was developed to evaluate this temperature. The numerical solution of steam condensate rate was compared with design value, and simultaneously, numerical solution of heat transfer coefficient was compared with Nusselt empirical formula value to verify the model validation. Further analysis on the model numerical solution shows that the partial differential equations of condensate film thickness can accurately describe the film characteristic which varies with geometry structure and declination of the tube. And the predicted value of vapour-liquid separation location agrees well with ACHE experimental operation. However, the numerical solution of the condensate sub-cooling temperature doesn't agree with actual measurement value very well. The result of this study is of great significance for optimization design of down-flow unit tubes, and also, it is meaningful for antifreezing protect of ACHE in winter.