Abstract: Take the unstable rock masses of the altitude concrete system of Yangfanggou hydropower station as a background. Based on investigation on spot and geological analysis, the boundary characteristics of the unstable rock-masses are analyzed; and the unstable rock-masses may to be failure through three modes: shear-sliding, rip-ropple and shear-falling is proposed; the stability classification standards of the different failure modes are established; the stability of the unstable rock-masses is analyzed by qualitative geological analysis and quantitative calculation. The results show that the nine dangerious rock-masses are highly likely to be failure under the action of rainfall or earthquake. Some protective measures are put forward, and the facts prove that the measures are successful and useful.