Based on the theoretical mechanism and the practical observation results of earth resistivity,two kinds of precursory anomalous changes of earth resistivity caused by tectonic activity are given in this paper. The first one is the long trend change related to the change of surrounding stress field,and the second one is the suddenly great change related to the fracture of rock crevice in the seismic source. In view of the two kinds of precursory anomalous variation of earth resistivity,taking the past observation data of five ground resistivity sites in Jiangsu Province as an example,using the normalized rate analysis of one-year window length,we proposed that the double variance of normalized rate value is as the anomalous index of ground resistivity. The R value test of prediction index is calculated as R=0.513,in corresponding to 97.5%confidence level R₀,R/R₀=1.9,indicating that the prediction results are sinificant.