Abstract The stray current generated by three subways in Guangzhou is monitored by an orthogonal electrode device, and its source is analyzed.Results show that the spontaneous potential generated by subway startup or braking exceeds 100 mV, while its background value is less than 3 mV at a distance of 1 km. The main period of spontaneous potential sequence is 119.2 s, and the other significant peaks are 51.16 s, 178.9 s and 358.5 s. The main energy of its power spectrum is below 200 s, which is significantly different from that without subway operation. Through template matching of the pulse details of the subway signal, we pick up the number of pulses close to the operating frequency of subways. Locating these pulses can identify the direction of source. We can develop filtering technology based on the data of this test and other in operation geomagnetic field and geomagnetic field data interfered by subway to effectively remove the stray current signals of geoelectric field and geomagnetic field.