Abstract: With the rapid development of urban rail transit, the direct current(DC) bias problem of power transformers brought by it has become more and more serious. Metro stray currents gradually become the main cause of DC bias magnetization of transformers in urban areas. Relevant design standards and monitoring results indicate that a metallic connection path exists between the grounding system of the metro and power grid, and this path may be one of the main factors affecting the stray current amplitude in the power grid. However, the path is rarely considered by existing studies and its effect on stray currents is unclear. Therefore, in order to analyze the influencing factors of stray currents invading the power grid and explore the influence of different path on the amplitude of stray currents, a coupling simulation model of metro and power grid is built, by comparing the effect of current amplitude with that of the earth medium as a propagation path, the degree of influence of the cable armored metal path is quantitatively analyzed, and it is found that the distribution of stray currents is significantly affected when a metal connection existed between the metro and the power grid, and the stray current amplitude of the intrusion power grid system is increased by 9 times. The research results show that the cable armor connection between grounding grids of two systems provide the good conductive paths for stray currents to invade the power grid.