Abstract: In detection of underground old kiln roadways and geological bodies by automatic earth resistivity method, the anomalies gradually become smoothly and fuzzy with their depth. Therefore, difference method of apparent resistivity is used to distinguish deep anomaly and to extract geological information included in measured data as much as possible. The results of finite element simulation and application show that difference technology adopted in common resistivity methods is available for automatic earth resistivity prospecting that belongs in resistivity method also.