Rock mass are subject to continuous changes induced by physical and chemical processes of natural and anthropic origin. Such changes affect their mechanical properties. To quantitatively evaluate the hydro-chemical corrosion, the theories of chemical thermodynamic and chemical kinetics are introduced. An index was proposed, which provides a criterion to judge whether the corrosion would be happened and indicated the reaction direction. Then based on theoretical and testing results, a hydro-chemical damage index was proposed. And a series of damage degrees of sandstone under different circumstances, which consider the variations of pH values, temperatures, ions species and ions concentrations, were calculated by chemical kinetics method. Based on the results, the sensitivity of influencing factors was discussed. According to the calculation and the theory of poromechanics or testing results, the mechanical properties of geotechnical materials under hydro-chemical corrosion process can be predicted, so can assess the risk of geotechnical engineering long-term stability.