Nonlinear finite element model (FEM) of NGW type planetary gear train of input stage transmission mechanism used in wind turbine boxes was established according to the nonlinear contact theory and finite element theory, for difference analysis of transient meshing characteristics of planetary gear pair between mating and nonmating surfaces. Transient meshing process of the gear pair was studied in dynamics experiments. The timehistory curve of stress of meshing tooth profile at different position was obtained for components of the gear train． The study results show that different position of meshing tooth profiles have similar stress variation history． The stress of internal planetary gear is bigger than that of the external planetary gear． On the basis of FEM, the comparison and analysis are carried out to find out the effect of some key working parameters on the transient contact of the mating surface and nonmating surface of the planetary gear pair, which provide instruction basis of transient optimal design for improving the meshing perofomance and reliability of a planetary gearing system．