Abstract: In order to improve the line transport capacity and vehicle running speed of urban rail transit, new urban rail transit lines with 25 kV AC power supply system have been planned in several major cities in China. The safety of personnel and equipment will be endangered by the leakage current of traction backflow system when it invades into the pipeline equipment surrounding these urban rail transit lines. The model of metro tunnel built by mining method is established, and the magnitude and distribution characteristics of leakage current under AC and DC power supply systems are compared. It is found that the leakage current of AC power supply system is much larger than that of DC power supply system, and its leakage current increases rapidly with the increase of traction power supply interval length. Rail-ground leakage resistance and soil resistivity are not the main factors affecting the leakage current. The leakage current of AC power supply system is mainly generated through space electromagnetic coupling, and reducing the intensity of electromagnetic coupling is the main way to reduce the leakage current.