Abstract: The geological phenomenon of rock-mass unloading and its engineering geological problems during the hydroelectric project construction are discussion in this paper. The discussion is based on the engineering practices of many high dam construction projects in the mountainous area of south-west China over last 40 years. The process of cognitive on the rock-mass unloading geological phenomena and engineering practice experience in the engineering geology area of this country are introducing briefly. A systematic rock mass relaxed zone according to the mechanistic mechanism types of unloading fissures is suggested. The relaxed fissures and depth unloading zone can be discovered behind the normal unloading zone. This zone is named as a depth fissures and depth relaxed zone. It is also discussed on the basis of a series typical examples. A mechanistic mechanism model for the depth fissures and depth relaxed zone is suggested. Some practice problems of hydroelectric project construction in connection with rock-mass unloading are discussed too.