Reducing the subjectivity of geological strength index GSI value and improving the accuracy of rock mechanics parameter value are crucial for surrounding rock stability. The sensitivity ranking of rock mass geological characteristic parameter’s to GSI was obtained by orthogonal test, and four parameters that had great influence on GSI were selected. The interval range of GSI was generalized by introducing the interval number of rock mass structural grade SR and structural plane surface SCR. The rock integrity coefficient K_Vand wave velocity ratio K’_V were introduced to perform linear interpolation for GSI interval range, so as to obtain accurate GSI value. Taking Qingjiagou tunnel as an example, the rock mass mechanical parameters estimated by this method were within the Ⅲ₂standard for surrounding rock. FLAC^3D was used to establish the tunnel excavation model.The relative error between the calculated vertical settlement value and the actual monitoring value was 1.5%, which verifies the reliability of this method.