This paper studied a mathematical method for separating sudden change anomalies from complicated background changes in earth-resistivity, and discussed how to recognize the sudden change anomalies in earth-resistivity due to earthquakes and sudden changes in interferences immidiately before the earthquakes taking the Jingtai earthquake (6.2) as an example. The result shows that for moderately strong or strong earthquakes which occur in the dense area of geoelectric stations, it seems to be possible to predict the epicenters and the times of occurrence using the method mentioned above immidiately before the earthquakes.