A variable inertial damper, whose viscous damping coefficient can be adjusted by changing the gap between inertia wheel and motor spindle, is designed in servomechanism of zero-drive hobbing machine and is directly attached on the motor spindle. The mathematical model of servo system with inertial damper is built. By using theoretical analysis and system simulation, it is demonstrated that the variable inertial damper with optimal damping coefficient and moment of inertia should lead to no resonance point in wider frequency range of exciting force. Therefore, its application in zero-drive hobbing machine makes this system not only achieve higher system stiffness to overcome load torque fluctuation, but also gain better stability.