Abstract: Detecting underground geological structures is one of the key considerations for such work as engineering geology, hydrology and geothermic survey by using geophysical prospecting methods. Generally the anomalies of radon measurement directly determine the attributes of underground geological structures, but without penetrating depth information. CSAMT, though incapable of attributes identification, has high resolution in exploring the positions, widths and attitudes of geological structures. Two practical examples show that combined exploration analysis can solve such problems as intruding contact of the granite body，fault contact between limestone in the anticline center and clastic rocks in anticline limbs and can also explore geological conditions controlling thermal springs developed on the boundary between soluble and non-soluble rocks. All these data can be used to evaluate engineering geological disasters，analyze geothermal formation and provide detailed information of underground geological structures.