The method for determining the geological strength index (GSI) with the surface condition and structure of rock mass mainly depends on their qualitative descriptions. There is much more subjectivity because the value is hard to get through the qualitative description. So

the quantitative indices are firstly used to determine GSI

such as process surface condition rate (SCR)

joint condition coefficient (Jc)

surface rate (SR) and quantitative block volume (Vb). SCR and Jc are used to reflect the surface condition. SR and Vb are used to reflect the structure of rock mass. In order to reflect the uncertainty of the quantitative indices

interval values are used to express them. Then the method for determining the interval values of GSI can be presented through the study of the determination of quantitative index values. Secondly

the interval combination method is presented to calculate the rock mass shear strength parameters according to the Hoek-Brown strength criterion. And the method for determining rock mass shear strength parameters based on interval theory and GSI is suggested. Finally

the method is used to determine the shear strength parameters of a hydropower station water intake tunnel in Hunan province. Compared with the existing analysis method

the results show that the method is reasonable and feasible. It provides a new approach to determine the shear strength parameters of rock mass

when the test data are less