Abstract: Considering the characteristics of underground cavern groups, a stability ranking system was introduced to denote the stability of rock masses surrounding the underground caverns. In this system four factors were used as indices including comprehensive quality level of rock mass, parameters of wedge (volume, shape and stability), displacement and failure zones, and intensity of rock burst. It also provided excavation schemes and supporting measures according to stability ranks. Then a case study was conducted using the ranking system to evaluate the surrounding rock mass stability of an underground cavern group at a planned hydropower station. The results showed that the stability rank of these caverns was classified as Grade II (In other words, the stability was good). The primary failure modes would be local wedge instability and light to moderate rock burst.