Based on the twoinertia characteristic of permanent magnet synchronous motor, the tuning of PID control parameters of motor speed loop was studied. The tuning rule of PID control parameters that made the system satisfy the evaluation criteria was studied when the natural frequency was 100 rad/s. To simplify the process, multiple variables were handled to three variables, frequency ratio x2, mass ratio x1, damping ratio x3. The fitting formula based on simulation data points was also given. Finally, the validity of the formula in shortening the time of parameter adjustment was verified by MATLAB.