The invention relates to a power line insulation monitoring method based on an instant fault. According to the method, on-line monitoring is carried out on a line through an on-site line selection device, and when the line breaks down, fault information such as the frequency, duration and intensity of the instant fault of the faulty line is recorded to work out the insulation coefficient. The insulation condition of the line can be accurately monitored under the condition of uninterruptible power supply. By means of the method, targeted power failure overhaul can be carried out on the line, so that the frequency of power failure caused by the fault is lowered, the duration of power failure is shortened, the power failure range is reduced, damage caused by regular preventable high-voltage tests on a cable is avoided, and unnecessary loss and resource waste are reduced.