Abstract: Simulation and analysis of the internal and external faults of a high-voltage AC transmission system shows that the physical boundary formed by the AC transmission line wave trap and the bus-to-ground capacitance has an attenuation effect on the high-frequency signal. This makes the high-frequency component of the fault voltage at the measuring terminal at external fault low, and the probability of mass distribution is relatively uniform in different time periods. When a fault occurs in the area, the high-frequency component of the fault voltage at the measuring terminal is high, and the probability of mass distribution is uneven in different time periods. Therefore, the non-uniformity Δα of the quality probability distribution of each subset on the multifractal set formed by each division is calculated using the single-side fault phase voltage of the line within the short-time window to identify the internal and external faults. This method can distinguish between internal and external faults by only using a single-ended voltage, is not affected by the channel, has high reliability, and has great engineering significance. Through a large number of PSCAD simulation experiments, it is concluded that this method has high reliability, strong ability to withstand transition resistance, and has a good ability to distinguish different fault types and remote high resistance.