Abstract: The geoelectrical resistivity observation is one of the most important methods in the study of earthquake precursor, and the characteristics of the stray current during subway operation in the geoelectrical resistivity observation are useful for analyzing the geoelectric observation data and the anomaly variation before earthquakes. Based on the study on the generation mechanism, the quantitatively results of the influence of stray current on geoelectrical resistivity observation were given in this paper. The result shows that the effect distance can reach dozens or even a hundred kilometers. Through monitoring the stray current in some geoelectrical resistivity observation stations around cities, such as Tongzhou station in Beijing, Qingguang, Baodi and Tanggu stations in Tianjin, Jiangning station in Jiangsu and Xinchengzi station in Liaoning, the amplitude and frequency range of the stray current signal as well as its spatial distribution characteristics were analyzed. It shows that the amplitude of the signal ranges from several to tens millivolt, and the period range is mainly from 50 s to 200 s. Due to the influence of stray current, the signal-to-noise ratio is decreased by 10 to 30 dB, and the accuracy of the observation data is more than 20 times worse than the specified requirement. According to the characteristics of the stray current propagation, several methods which can be used to suppress the signal were proposed in this paper, and it will provide a foundation for the identification of anomaly information from the observation data and anti-interference technology study.