There is an active impact of the sending system during a commutation failure caused by an AC fault on the inverter side. Based on the DC equivalent circuit during the commutation failure, the dominant factor of the active power fluctuation of the rectifier after the commutation failure is clarified. The response mechanism of each electrical quantity and control quantity on the rectifier side is analyzed by considering the fluctuation characteristics of the DC voltage during the commutation failure. This is to determine the relationship between the transient response of each DC electrical quantity and the control quantity on the rectifier side and the transmission active extreme value moment of the sending system. Then, based on the tensile operation circuit during the commutation failure, the time-domain expressions of each electrical quantity of the sending end system after the commutation failure are deduced and established. The transient active power modeling of the rectifier is realized. By dividing the time interval to optimize and improve the low-voltage current limiting link, the DC voltage time-domain expression is used to compensate for the DC current command value, an active support strategy based on the transmission active power transient extreme moment is proposed. Finally, the simulation results of PSCAD and CIGRE-HVDC verify the correctness of the theoretical analysis and the effectiveness of the active support strategy.