The load sharing characteristics of a planetary gear train is one of its main performance. Many researches at home and abroad have shown that the floating effect of components in a planetary gear train has an important influence on load sharing characteristics of the system. In this paper,the relationship between sun gear and spline on the input shaft is considered. Also,friction of inner and outer teeth in splined connection as well as constraint floating of sun gear are simulated. The nonlinear opposite supporting force of the spline on the input shaft to the sun gear is established. Taken a 5-planetary gear transmission system as an example,its load sharing coefficients with different radial clearance of a spline are calculated by the lumped-mass method. The research result provides a basis for load sharing design of planetary gear trains.