During the operation, the change of moment of inertia has a negative influence on permanent magnet synchronous motor (PMSM) servo system. Therefore, the quantitative feedback theory (QFT) is used to design PID controller. The moment of inertia is regarded as the parameter variation, and the uncertainty model of the controlled plant is obtained. Then the expected performance index is set. The PID controller is synthesized on Nichols diagram by QFT. Simulation and experimental results show that the proposed method can effectively improve the influence of moment of inertia change of PMSM servo system and enhance the robustness of the system. Finally, the comparison with PI controller shows its superiority.