There is a problem of commutation failure (CF) at the inverter side caused by sending end AC system faults. First, based on the mechanism of reactive power interaction in AC-DC systems and the expression of extinction angle, the operational characteristics at the inverter side are analyzed in depth, and the variation laws of DC current rise speed and amplitude under different fault severity are clarified. It is concluded that the excessive amplitude of DC current in a slight fault and the excessive rise speed of DC current in a severe fault are the main reasons leading to CF. Secondly, the reactive power consumed by the rectifier order value with different fault severity is quantitatively calculated, the expression of DC current and voltage and the reactive power consumed by the rectifier is proposed. A CF prevention strategy based on rectifier reactive power control is proposed. Finally, the simulation results based on PSCAD/EMTDC verify the correctness of the mechanism and the effectiveness of the proposed prevention strategy.