The stray current generated by the subway could cause DC magnetic bias in the transformers of the urban power system, threatening the safe and stable operation of the transformer and the power system. In order to solve this problem, a platform monitoring the stray current invading the substation and the DC magnetic bias characteristic of the transformer is developed. According to the monitoring result, the characteristics of the amplitude, distribution and flow direction of the intrusive stray current are analyzed, and the influence of the time period, the subway operation mode, the substation location and the number of nearby subway lines on the DC bias characteristic of the transformer are studied. The results show that the stray current through the neutral point of the transformer could cause severe DC magnetic bias problem, and there might be a metallic transmission path for the stray current between the subway system and the power system. Thus, it is necessary to carry out targeted research on relevant prevention measures from the source in conjunction with the planning and site selection of the substation.