Abstract: The series fault arc caused by poor wire contact and other reasons in power lines is difficult to extinguish and detect,and the generated high temperature can easily lead to secondary disasters such as fires.In order to deal with the problem that the steady state current of the series arc fault is small,which leads to the difficulty of detection,the arc high-frequency fault feature is widely concerned and applied,but the mechanism of the fault feature is still unclear.A fault arc numerical model was established.It is found that the dynamic change process of cathode spots is one of the main reasons producing the high-frequency characteristics after fault.However,the traditional arc model regards the cathode surface process as a stable change and cannot characterize its high-frequency characteristics. The dynamic change process of the cathode spot was simulated by using the change of the radius size of the cathode spot,and a model considering the new boundary conditions of hot electron emission was established.The comparison of the simulation and metered experimental waveforms proves the correctness of the proposed arc model and the founded arc high frequency characteristics.