In order to study the intrusion range and degree of subway stray current into the AC power grid, a mathematical model of stray current which is based on the degradation of insulation performance of rail insulation fasteners under actual operation is established, and the stray current leakage of each sleeper is derived. The subway stray current is discretized into a finite point current source according to the number of sleepers, and the Green's function is solved based on the complex image method. The ground potential distribution calculation model caused by the subway stray current is also established, and the commercial grounding software CDEGS based on the boundary element is deployed to verify the method accuracy. The influence of traditional sensitive factors, such as stray current leakage and soil structure, on the ground potential distribution along the urban rail line and the surrounding ground potential fluctuations under the operation of multiple trains in a multi?line transportation hub area are discussed.