The infinite block stability of rock mass surrounding a large underground excavation is studied in this paper through an actual case of a large powerstation in southwest. The study indicates that infinite block stabili-ty is very important for the stability of rock mass surrounding underground excavation. Blocks can be classified into infinite block and, finite block. Infinity block is regarded as stability at present. However, infinite block may lose its stability under some conditions and its instability may be a new failure form of rock mass surround-
ing underground excavation. Furthermore the boundary condition of infinite block instability is defined. Finally,the method to study infinite block is proposed and study trend of block stability is discussed.