Line-commuted converter-based high voltage direct current (LCC-HVDC) is prone to commutation failure, causing transient overvoltage at the AC bus of the rectifier station. Given that AC transient overvoltage can threaten the safe operation of the sending-end system, an AC transient overvoltage control method of the LCC-HVDC sending-end system is proposed. First, the influence of the phase locking error of the rectifier station on the firing angle control is analyzed by analyzing the characteristics of the transient overvoltage during commutation failure, and the influence of the phase locking error on the AC transient overvoltage of the rectifier station is quantified. Then, the threshold DC current calculation method for compensating the unbalanced reactive power of the rectifier station is proposed based on the reactive balance relationship between the sending and receiving ends. Next, a calculation method of firing angle command value for compensating the dynamic error of firing angle is proposed. The control method for sending-end AC transient overvoltage based on the compensation of unbalanced reactive power and firing angle error is proposed. Finally, simulation cases demonstrate that the AC transient overvoltage of LCC-HVDC sending-end system can be effectively suppressed by the proposed method.