Abstract: The first thing in the stability analysis of underground opennings is the determination of the deformation failure-movement type of rock masses. In this paper five main types of deformation failure-movement has been mentioned, according to different combinations of the four rock mechanics factors. Thus, primary appreciation of stability and selection of mechanical model for the rock mass can be made in design work. Some methods of stability analysis for rock mass composed of rock blocks has been discussed, referring to the underground power station of The Three Gorges project as an example. Planes of discontinuity should be simplified according to their relative size to the dimensions of the cavity. The influence of the stress component along the axis of the cavity on the stability of rock blocks has been emphasized. In those cases when finite element method in three-dimensions can not be used, calculations may be carried out by two-dimensional finite element method in combination with the method of the whole space projection on equatorial plane. When the orientation of the axis of cavity is to be determined, not only the direction of the maximum in-situ stress but also the azimuth of planes of discontinuity should be considered.