Regarding the gear in 2K-V planetary transmission mechanism as the research object and based on the theory of gear meshing and the heat transfer， the changing rule of the relative sliding speed and contact stress of tooth surface at different meshing positions is analyzed. The boundary conditions of steady-state temperature field are accurately calculated， including the friction heat flow within one meshing cycle and the convection heat transfer coefficient of different tooth surfaces. A single-tooth model is established and ANSYS Workbench for finite element analysis of temperature field is imported into. By dividing the meshing surface into several strip areas along the tooth width， the friction heat flow at different meshing positions can be loaded and the steady-state temperature field distribution of gear is obtained. The influence of the number of strip areas on the temperature field is analyzed. The results show that the temperature field tends to be stable when the number of strip areas is more than 50.