Abstract: Classification of surrounding rock is the key issue in the design and construction of underground caverns in hydropower projects, and it′s significant to properly determine the grade of rock mass. By using HC classification,BQ classification, Q classification and RMR classification methods, it studied the grades of rock mass of underground caverns and a correlation analysis on scores of various classification systems was conducted by the regression method, with the results show that it is more beneficial to rational grading of surrounding rock by comprehensive using HC classification, BQ classification, Q classification and RMR classification for the large-span underground cavern;the results of HC classification agree well with that of RMR classification if the lumpiness and integrity of rock mass are governed by structural surface.