With engineering examples, this paper describes the application of the ultra-high density resistivity to the investigation of karst caves and fracture zones, summarized the points for attention during the data acquisition, and made a comparison between the achievements obtained and the drilling data. The results show that this method is fairly effective in detection of the distribution, sizes, buried depths and development of karst caves and fracture zones, and can thus serve as a geophysical method for improving the precision of engineering exploration.