The performance of servo control system is limited by mechanical resonance, so the engineers need to readjust the controller parameters in practical application. To improve the accuracy of PID parameters, the mathematical model of resonance mode was established before the controller of position loop of servo system was designed, and the resonance mode was regarded as a part of the controlled object of position loop of servo system. Secondly, a two-degree-of-freedom controller for the servo system was designed according to the mathematical model of the controlled object including the resonant mode. The feedforward controller was designed by invariance principle, and the feedback controller was designed by the proposed five-order pole assignment method. Finally, the proposed new servo system design method was simulated.The simulation results prove the effectiveness of the proposed method.