Abstract: ased on the field investigations and from results of chemical experiments, the authors studied the complicated buried karst and deep underground confined water, clarified the formation conditions of karst and underground confined water beneath a dam foundation, and divided the underground water system into two parts: the first one is an underground water system in cracks or karst; the other is underground confined water system. Then it is pointed out that there is no hydraulic relation between the two systems. These results will be beneficial to selection of dam sites and engineering construction.