“10·04” commutation failure of 500 kV Tianshengqiao-Guangzhou HVDC is simulated and analyzed with RTDS tool, and the influence of AC filter switching operation on the transient process and commutation failure of Tianshengqiao-Guangzhou HVDC are systemically studied.The main conclusions are as follows: “10·04” commutation failure of Tianshengqiao-Guangzhou HVDC is caused by the 90° delayed closing of the AC filter after AC current crosses zero point, which leads to AC voltage distortion; the effective short-circuit-ratio of Guangzhou converter station is small, if AC filter input delay angle is too large, AC voltage will be distorted and result in commutation failure, but due to the short voltage distortion duration, this will not cause continuous commutation failure or DC block; in case of serious AC bus voltage distortion caused by improper AC filter switching operation, with the increase of DC power, the more DC voltage drops, the more significantly DC current increases, the more converter transformer side current reduces, the more easily commutation failure occurs in DC system; reducing the dispersing of AC Filter switching operation and ensuring an ideal angle can effectively prevent commutation failure caused by AC filter large angle closing.