Abstract: ZhaoQing Bridge, which is a main line connecting the ZhaoQing City and pearl Delta area, is situated on the Xi Jiang river catchment area in Guang Dong province. Their main pile piers are all situated on corroded limestone & marble cavity or above the rupture fragmentation. The one borel hole for one pile on every pier, a tradiational method for collecting geological data in Bridge foundation, is hardly possible to find out the cativities in corroded limestone and marble basic rock. The Cross Well Electromagnetic Tomography Technique is just possible for making up this shortage . We had used this technique in five main pile piers of ZhaoQing Bridge, by laying out crisscross prospecting profiles in each pile pier, constructing three dimension cavities distribution map in underground corroded limestone & marble rock through these crisscross profiles, figure out the cavities size & extension in difference direction .