Abstract: In this paper,the dynamical model which was coupled of vertical vibration,horizontal vibration and torsional vibration was established. The vehicles transmission system was considered as a multi branch system.The spectrum of terrain roughness, engine angular velocity and drawbar pull were three excitation sources of the system. With a self compiled program, the root mean square and power spectrum of the responses can be simulated and calculated.Through experiments, it's verified that the dynamical model is correct.