Abstract: The trend variations in the earth-resistivity over one year scale are observed in large quantities. This trend variation is regional and synchronous. In order to study this phenomenon, this paper analyzes the reasons for the trend variation of resistivity from the view points of the relationship between water level and ground resistivity, and between ground level and resistivity, as well as the earth tide phenomenon of ground resistivity through the theory, experiment and example analysis. It is considered that the trend variation is independent of the observation system or the water level (non-tectonic water level), but relate to structural change. The rising and falling trend respectively reflect the compressional and extensional variation of stress field around. Furthermore, taking the stations Gaoyou and Chengdu as examples, the characteristics of the resistivity trend are analyzed based on the relationship between the surrounding earthquakes and the resistivity. The result shows that the uptrend turning has short-term predictive implication, which can provide a reference for the research on the long-term trend of resistivity. The future earthquake prediction needs to be integrated with the observation results of each station, which can effectively determine the direction and the relative amplitude of regional stress field, so as to give some significant results of earthquake precursor and prediction.