The stray current of urban rail transit flows into the grounded transformer winding of the urban power network, resulting in the DC magnetic bias of the transformer, which affects the safe and stable operation of the transformer and even the power grid. In view of this phenomenon, through large-scale field monitoring, the DC magnetic bias monitoring data of 11 substations of 500 kV and 220 kV in East China’s megacity are analyzed. By comparing the operation characteristics of rail trains, the dynamic fluctuation characteristics of stray current are summarized and analyzed. The results show that the dynamic characteristics of transformer stray current are closely related to the operation characteristics of the trains of urban rail transit, with obvious volatility, intermittency and periodicity. By analyzing the geographic information distribution of stray current and its related power grid structure, as well as its positive and negative proportion characteristics, the electrical circuit principles about the urban power grid’s absorption, propagation and dispersion of the stray current are obtained. Finally, based on the continuous monitoring data of stray current, the corresponding statistical method is proposed.