Abstract: For two inertia system, the traditional design uses representative pole configurations, in order to improve the design degree of freedom, the parameter plane method is introduced to overcome difficulty in parameter adjustment, and design on PI control system is presented. Aiming at the characteristics of underdamping in small inertia ratio systems, a low pass filter is introduced to suppress mechanical resonance. By analysis and simulation, it is found that the damping characteristics of the system is closely related to the inertia ratio, and the filter can improve the damping characteristics of the system effectively. The feasibility and usefulness on extended parameter plane method for adjusting the three parameter is shown. Finally, the effectiveness of theoretical analysis and design are verified by experiments.