To investigate the defect location of the 10 kV cable network, a new method of insulation diagnosis based on different response characteristics of cable under impulse voltage is presented. Simulation model of 10 kV cable network under power frequency voltage superimposed of impulse voltage is constructed in PSCAD/EMTDC program. The transient propagation characteristic of the current and voltage on the main cable feeder is analyzed. The change of the current is used to present the defect of the cable and judge the location of the defect. The simulation results show that the current on the two sides of the impulse source is bipolar, while the voltage is single polar. After the defect is set in a cable, the current on the two sides of the test cable is different from the current before the defect is set. Comparing the change of the current waveform of each node on the main feeder with and without defect, the location of the defect can be estimated.