Abstract: The drop in AC voltage and the increase in DC current after an AC bus fault on the inverter side in LCC-HVDC are the main reasons that cause commutation failure. In order to obtain the turn-off characteristics of the converter valve thyristor under the combined effect of post-fault AC voltage and DC current transient behavior, an approximate analytical method for the maximum DC current under different AC voltage dips after a fault in the receiving AC system is proposed. Then, the equivalent circuit of the commutation and turn-off process of the thyristor is established in the SABER platform, and the post-fault thyristor turn-off characteristics are researched by comprehensively considering the changes in AC voltage and DC current. Based on the analysis model of carrier migration recovery characteristics, the residual charge inside the thyristor at each stage is calculated, and the turn-off characteristics of the thyristor are quantitatively evaluated. Finally, a comparative analysis is conducted on the thyristor turn-off characteristics, considering only the AC voltage drop, only the DC current increase, and the combined effect of both. The results indicate that, with the increasing severity of faults, the recovery time of the thyristor's blocking capability (the critical turn-off angle) exhibits a decreasing trend. However, the commutation time and total turn-off time significantly increase, posing a potential risk of commutation failure in the converter valve. Considering only the AC voltage drop or the DC current rise after a fault leads to an overestimation of the critical turn-off angle. On the one hand, this may impose higher turn-off requirements on the system. While on the other hand, it is likely to misclassify scenarios experiencing no commutation failure as commutation failures. Considering the dynamic variations of both AC voltage and DC current provides more accurate thyristor turn-off characteristics.