Abstract: The proportion of high-voltage power cables in the transmission channels of urban power grids has increased year by year,and once a failure occurs,it will cause significant losses. The article details the terminal fault of the main variable 35 kV low-side cable that occurred in a 500 kV substation,calculates the capacitance current of the low-voltage side system combined with the harmonic current on-line monitoring system,and analyzes the fault development process of the cable combined with the fault recording waveform at the time of protection action. By dissecting the cable terminal,the fault causes are qualitatively analyzed,and then suggestions are put forward for the safe operation and fault prevention of high-voltage cable line.