The stray current generated by the massive operation of subway transit will affect nearby power transformers by DC bias, threaten the safe and stable operation of transformers and AC grid systems. In order to solve such problems, the static distribution model of subway stray current, the multi-layer block soil model of Shenzhen and the simulation model of Shenzhen AC power grid are established for simulation analysis. The variation of the ground potential of the AC power grid substation and the distribution characteristics of the neutral point DC current of the transformer caused by the stray current are calculated and analyzed. The results show that the change of ground potential and DC current through transformer neutral point is related to the operating conditions of the subway. The distance between the substation and the dense area of the subway line and soil resistivity are the main influencing factors. For the autotransformer, combined with test results, it is pointed out that the installation of the capacitive blocking device at only the DC over-standard site cannot eliminate such DC magnetic bias problems, and it is necessary to properly arrange the blocking device from the perspective of the entire regional power grid.