For two branches of power transmission gear,the bending- torsion- shaft coupling dynamics model and the corresponding nonlinear dynamical equations are established in this paper,with considering the varying mesh stiffness,backlash and the tooth profile modification as a time-varying backlash. Numerical solution of the equations into solving relationship obtained with the gear tooth profile modification Vice dynamic load factor,found a reasonable modification allows the separation power split tooth gear does not occur the teeth back impact. Using different modification the amount of each gear pair dynamic load factor peak contour,the optimum amount of modification,so that dynamic load factor smallest gear. Numerical results show that a reasonable tooth profile modification,the maximum dynamic load factor of the two branches of power transmission gear can be reduced by 3. 6 to 1. 6.