Abstract: The distribution of soluble carbonate in the dam foundation of Cuijiaying Hydroproject (Z2d siliceous dolomite, dolomite, etc), obvious karst phenomenon, existence of leakage and uneven foundation settlement (or karst collapse) and other key engineering geological problems have great impact on the project. Through geophysical prospecting, drilling, mineralization analysis, electromagnetic perspective and water pressure test the characteristics and laws of karst are discovered as follows: (1) There is no large-scale karst foundation pipeline system, poor connectivity and cave dissolution porosity; (2)Dam may be a weak dissolution of carbonate lava rocks, and the dolomite karst is easier to be dissolved than the siliceous dolomite; (3) Karst regularly develops along the rock level, tectonic fissures and fault location; (4) The extent of karst development decrease with the elevation the karst at depth of above 21 m is strongly dissolved, and the karst below depth of 21 m is weakly dissolved; (5) The extent of corrosion increases with the strong permeability of rock. It provides the geological basis for the design and construction departments and has certain reference value.