Abstract: A set of separate heat pipe (SHP) with water as medium was built. The flow and condensation of vapor with non-condensable and high partial pressure air in the SHP were measured in the experiment. The physical model of the flow and condensation of vapor-air mixture was studied and the related numeral model was founded. Except for the mass, momentum and energy conservation equations, the diffusion and condensation equations were added to calculate the distribution of the mixture in the mathematical model. The numerical result agrees well with the experimental data, and the deviation is between 8% and 15%.