There is stray current in the subway tunnel whether it is AC or DC. Reducing the influence of stray current is a problem that cannot be ignored in the process of subway design and construction. In this paper, the distribution law of stray current in subway tunnels under different power supply systems is studied. The results show that the stray current generated under the AC power supply system is much larger than the stray current under the DC system; the return lines of the two power supply modes have a certain shunting effect, so that the rail current decreases by about 1/5; In the DC power supply mode, the addition of the return line cannot effectively reduce the generation of stray current. In the AC power supply mode, the increase of the stray current after adding the return line has a certain effect.