Three kinds of typical variations of earth resistivity at Baochang Station are analyzed, which includes daily, annual and secular variations. The result shows that there exists significantly daily variation morphology in the curve of earth resistivity hour-point values, which is caused by unstable insulating properties of transmission lines. The annual variation of earth resistivity is mainly caused by meteorological factors including temperature and rainfall. There exists secular oblique-type variation only in the curve of earth resistivity in NS direction, which is not related with the secular variation of water level, temperature, and rainfall etc.. However it is mainly caused by the long-time prolonged action of stress in EW direction, which is not corresponding with the earthquake.