Based on Newton’s second law,the bend-torsion-shaft coupling nonlinear dynamic model and equations of power split gear transmission system are established.According to the principle of tooth profile modification,the tooth profile modification is considered as time-varying gear backlash function acting along the line of action.Then the dynamic functions are solved by using Runge-Kutta numerical method.After analyzing the effect of tooth profile modification quantity(TPMQ)and relative tooth profile modification length(TPML)to the nonlinear dynamic characteristics of power split gear transmission,the following conclusions are drawn:1The TPMQ of a certain stage transmission affects the vibration of its own stage more significantly than the other stage,and the coupling effect between two stages can be ignored usually in the modification design;2If the first stage TPMLs are less than 0.3,the influence of the first stage TPMLs to the first stage transmission vibration is much more greatly than the influence of the second stage TPMLs to the first stage transmission vibration,or else both the first and second stage TPMLs affect the first stage transmission vibration largely.The same is true for the second stage TPMLs,and the cutoff value is 0.2;3The TPMQ affects the vibration of power split gear transmission system more principally than the TPML,and should be top-priority in the modification design.:power split gear transmission system;tooth profile modification;nonlinear dynamic characteristics; Based on Newton’s second law, the bend-torsion-shaft coupling nonlinear dynamic model and equations of power split gear transmission system are. Established to the principle of tooth profile modification, the tooth profile modification is considered as time-varying gear backlash function acting along the line of action. The dynamic functions are solved by using Runge-Kutta numerical method. After analyzing the effect of tooth profile modification quantity (TPMQ) and relative tooth profile modification length (TPML) to the nonlinear dynamic characteristics of power split gear transmission, the following conclusions are drawn: 1 The TPMQ of a certain stage transmission affects the vibration of its own stage more significantly than the other stage, and the coupling effect between two stages can be ignored usually in the modification design; 2If the first stage TPMLs are less than 0.3, the influence of the first stage TPMLs to the first stage transmission vibration is much more greatly tha n the influence of the second stage TPMLs to the first stage transmission vibration, or else both first and second stages TPMLs affect the first stage transmission vibration largely. the same is true for the second stage TPMLs, and the cutoff value is 0.2; 3The TPMQ affects the vibration of power split gear transmission system more principally than the TPML, and should be top-priority in the modification design.: Power split gear transmission system; tooth profile modification; nonlinear dynamic characteristics;