Abstract: The geological conditions of the Main Dam of Xiaolangdi Multipurpose Dam Project are very complicated,hence the seepage prevention and foundation treatment works are very difficult to handle.The construction units adopted new techniques,new methods,new workmanships and advanced complete large construction equipment. They overcame the constriction technical problems,shortened the direct duration for the construction and meanwhile,they created the new construction record in China,and provided some useful references for the similar works.