During the process of the commutation failure in power grid, a large amount of dynamic reactive power are consumed. Once a commutation failure occurs, it affects the safe operation of the grideasily. Under the background, anreservereactive poweroptimization for AC-DC hybrid power gridsisproposed to reduce the risk of accidental DC commutation failure propagation. Firstly, a differential algebraic equation for the AC / DC hybrid system is established. The multi-order trajectory sensitivity of the system is obtainedon the basis of the first-order trajectory sensitivity. According to the current operating conditions of the power system, a dynamic reactive power reserve optimization model including a generator and a reactor is then established, and the optimal solution of the dynamic reactive power reserve optimization is solved. Finally, a practical system ismodeledto verify the feasibility and effectiveness of the method.