The metro stray current leaked during the train operation causes the surface potential fluctuation, resulting nearby transformer DC bias. Firstly, a equivalent resistance network model was established to realize actual time numerical calculation of stray current. Then, the stray current field equation was further derived, and the dynamic simulation of the geopotential potential distribution was realized during the train operation. Based on the potential distribution caused by stray current and the equivalent DC resistance model of AC power grid, the numerical calculation of DC bias current and excitation current of grounding transformer was realized. The index of neutral point current and waveform distortion rate is established, so the effective evaluation of the transformer DC bias could be realized. The simulation results show that the operating conditions and distance between the metro train have obvious effects on the transformer DC bias, so formulating a reasonable metro operation strategy could effectively restrain transformer DC bias interference for nearby substations.