The metro stray current can flow through the neutral point of the transformer from the grounding grid in the soil, and then cause the transformer DC bias phenomenon. In order to effectively evaluate the influence of metro stray currents on transformers, a resistance model for the transformer DC bias caused by the stray current is established by considering the traction current in metro, the "railcurrent drainage netearth" metro system structure, and the DC bias current loop. Then the dynamic equation of transformer neutral point current with time and metro train position is established, and the neutral point current can be solved by the LU decomposition. The software CDEGS is utilized to build the train and transformer simulation model. The transformer neutral point current between the simulation results and the calculation results of the proposed modeling method are comparative analyzed, and the model error is small. Therefore, it can be effectively applied to the evaluation of transformers affected by metro stray current.