Taking a geardriving shaft system as the object in this paper, an equivalent model of the target system with shafting torsional vibration is developed. At first, the free vibration analysis is studied to obtain the natural frequencies of the torsional vibration. Then, the gear variable stiffness and the gear dynamic excitation are simulated by the software Matlab. Moreover, the Newmark Step Integration Method is applied to calculate the forced vibration response of the shaft torsional vibration under gear dynamic excitation or extraapplied load torque or both. It is concluded from the comparison of the results that gear dynamic excitation cannot be neglected in the shafting torsional vibration.