Abstract: With constant changes in power system and source structure. The dynamic stability of central China power grid has undergone a complex process of evolution and continues to descend in recent years, which has been the shortcoming of power system operation. This paper summarized the evolutional process and features of dynamic stability of central China power grid. Then, the influence of power grid structure, operation mode and generator excitation system parameters on dynamic stability is systematically studied. Based on practical situation, the dynamic stability defense system including preventive control and emergency control is designed to improve the dynamic stability of power system from optimization of the unit excitation control system and operation modes, disturbance source location and HVDC supplementary control. The effectiveness of the defense system has been proved through numerical simulation and engineering practice.