The invention provides a parallel compensation system and method for preventing a commutation failure of a DC power transmission system. The system comprises a reactive compensator and a controller, wherein the controller is used for controlling the reactive compensator. The method comprises the following steps of detecting a three-phase voltage of an AC bus on an inversion side of the DC power transmission system; analyzing and distinguishing a three-phase voltage signal value, sending an analysis result to a fault distinguishing unit, and comparing and calculating a predetermined voltage setting value of the AC bus with the three-phase voltage signal value; triggering a power switching tube of the reactive compensator according to a three-phase reactive current command signal; separately controlling a reactive current component output from each phase of the reactive compensator, and compensating the voltage drop of the bus; and controlling an active current component output from each phase of the reactive compensator, and maintaining constant voltage of a sub module capacitor. With the system and the method, provided by the invention, the voltage drop of a power grid can be effectively and accurately compensated, reactive compensation control on a negative sequence and a zero sequence is achieved, the commutation failure generation is prevented, and effective running of the high-voltage DC power transmission system during commutation of the power grid is further ensured.