In HVDC systems, because of switching frequently and bad working conditions, AC filter circuit breaker can occur the insulation breakdown fault easily. Based on a real case of AC filter arc fault, firstly, the process of accident is described briefly, and then when the circuit breaker is in different stages such as the normal condition, the breakdown fault stage, the continuous arc burning and extinction stage and spark-over totally, the corresponding arc voltages and currents in different stages are compared and analyzed, besides, the current harmonics and the total harmonic distortion rates when circuit breaker state operates normally are calculated. At last, through analyzing the dynamic record waveform of arc voltages and currents in different stages and the strip inspection of on-site circuit breaker, the reasons of arc fault are explained, and some relevant suggestions are given, which provides references for production and operating maintenance of circuit breaker used in AC filter. This work is supported by National Natural Science Foundation of China (No. 51477084) and Science and Technology Project of State Grid Shanghai Electric Power Company (No. SGSHJX00ZSJS1600747).