Based on the fact that the distance between the heat exchange surfaces has little influence on the separation efficiency in the short-path distillation, a hypothesis that the heat and mass transfer process in the transition region is controlled by heat convection is proposed; the gas-liquid state in the transition zone was calculated by numerical simulation experiment. The results show that the gas-liquid volume fraction and temperature fluctuation in the evaporation and condensation process is unstable, while the gas-liquid volume fraction and temperature fluctuation in the transition zone is stable in the short-path distillation process. It can be concluded that in unsteady thermodynamics, the transition zone is a stable convective heat transfer process that is not affected by the distance between heat transfer surfaces. Thus, under ideal conditions, the continuous extension of the transition region has little effect on the distillation efficiency.