Abstract: In order to solve the shortcoming that the existing frequency domain reflection method can only locate and detect cable defects in offline situations, this paper proposes an online cable defect location method based on frequency domain reflection method and electromagnetic coupling technology. Firstly, based on the cable distribution parameter model, the cables with local defects are simulated and modeled, and the influence of the defect points of different capacitance changes and different test frequencies on the positioning effect of the cable defects is analyzed. Then the influence of the sensor structure on the signal injection efficiency was explored. Finally, it was verified on 105 m and 500 m power cables in the laboratory. In the case of offline testing, the load resistance of the 105 m cable end was changed to simulate different cable loads. The line condition, and analyze its influence on the reflection intensity at the local defect and the end of the cable. In the case of live test, the 500 m cable is energized through the series resonance platform, and the cable is tested using the online positioning method proposed in this article to verify the effectiveness of the method proposed in this article. The simulation and experimental results show that: compared with the cable offline positioning test method, this method can realize the online positioning of the cable defect point when the cable is energized, and has higher positioning accuracy and recognition sensitivity.