Abstract：A lot of investigation shows that these unstable rock masses can be classified into four failure modes：whole slide shear failure mode；rock block landslide failure mode；whole shear fall failure mode；compress-rip- slide failure mode. Unload cracks of forming unstable rock masses failure boundary don¢t store water usually；so the main loads to cause unstable rock masses failing in dam region are weight of unstable rock，seismic load. There are two types of load combination，the first one is only weight，the second one is weight and seismic load. According to limit equilibrium theory，the authors put forward different calculation methods of stability and criterion of stability state class for different failure mode. The limit equilibrium theory not only emphasizes that correlative intensity parameters must be exact，and correctly measure geometry parameters of unstable rock masses；the latter are commonly difficult，utilizing the measuring technique of three laser scan to get these credible geometry parameters. The authors put forward corresponding measures of prevention and monitoring unstable rock masses of different scales and stability states too.
