Abstract: The interference current of rail transit causes DC bias of transformers in urban power systems, which is not conducive to the high robustness of transformers and power networks. To address this challenge, a global positioning system based online monitoring system for substation interference current entry and transformer bias was invented. Through online data, the size, arrangement, and flow direction of intrusion interference current were studied, and the interference of particle size such as timetable, rail transit scheduling, substation address, and the number of surrounding rail transit on the DC bias characteristics of transformers was analyzed. Facts have confirmed that the DC bias anomaly caused by the interference current entering the neutral point of the main transformer is prominent, and there may be a diffusion mode of interference current conductivity established between the rail transit and the power system. Therefore, it is necessary to comprehensively focus on relevant rectification measures based on the selection of substation address.