The transmission system of vehicle what’s input source is the torque of engine are studied. The dynamic model is built by using lumped parameter method considered coupled effect of bending and torsional. The response sensitivity equations derived from the dynamic equations are proposed by the direct derivation method. Then, the solutions of the response sensitivity equations are turned into the sensitivity of actual physical quantities such as additional torque and reaction force of bearing with respect to the design parameters. Finally, the solutions of dynamic equations and response sensitivity equations are achieved by fixed-step RK4. Some useful laws are achieved by comparing different results of sensitivity equations. The sensitivity of coupled effect model of bending and torsional makes some adjustment when comparing previous model. There is some relation between the vibration energy of the location place and the sensitivity of the force with respect to design parameters. The bigger vibration energy means the bigger value of sensitivity. The sensitivity result is meaningful for modification of structure and optimization of parameters.