The G23 mylonitization and fractured belt located in the dam-foundation of a hydropower station is a main engineering geological problem of the project. Its genesis mechanism has been analyzed based on the characteristics of the new tectonic movement and the tectonic system. With the X-ray diffraction and scanning micrograph analysis

the main mineral components and microstructures of either the fractured rock or the phyllonite are identified. With the conventional triaxial compression testes

the stress-strain characteristic of G23 belt rock is examined. In conjunction with site specific analyses

the pre-existing state of rock mass in G23 belt is expounded. At last

based on physico-mechanical properties

the mass rock’s engineering characteristics are estimated; the influences of the G23 belt are discussed; and the main problems which should be study further are put forward.