Abstract：The development of rock mechanical parameters estimation methods using rock mass mechanical parameters in Hoek-Brown criterion as well as measures of improvement in the process are traced to the source. However，geological strength index(GSI)，the newest method created by E. Hoek，has two deficiencies：depending on qualitative description completely and subjective experiences by individuals. By further study，it is discovered that the physical factor which determines geological strength index is the type of rock structure, and the chemical determining factor is weathering condition. Then，quantitative description of rock structure is achieved by introducing rock-mass block index based on drilled rock samples of different complete lengths. Meanwhile，absolute weathering index，which denotes rational index of weathering condition，is put forward based on the fact that rock weathering is the process of lithology changing caused by original mineral hydrolyzation and mineral alteration led by leaching out；and the calculation method is studied as well. Then，through organic combination of rock mass block index and absolute weathering index in the general sector scope table，the quantitative method of GSI is constituted. Finally，the mechanical parameters of surrounding rock in a highway tunnel are estimated，such as deformation modulus，tensile，compressive and shearing strength etc.，and the pitch of cave arch is calculated as well. Relative error is around 14% compared with measured data；so the feasibility and effectiveness of the quantitative method are confirmed.
