Abstract: Granite is often selected as a foundation for dams due to its good physical and mechanical properties. However, hydrothermal alteration induces significant changes in the mineral composition, structure, texture, chemical constituents, and physico-mechanical properties of deep-seated rocks. Therefore, the study of hydrothermal alteration characteristics in deep-seated granite holds paramount significance for hydraulic engineering constructions. The case of the Pingjiang Pumped Storage Power Station reservoir area in Hunan Fushou Mountain exhibits widespread exposure of Late Mesozoic granite. The main rock type is monzogranite. Surveys have indicated varying degrees of argillization and chloritization within the granite, showing evident zonation of alteration intensity. In this research, an investigation is undertaken into the mineralogical composition, geochemical constituents, and mechanical indicators of granite rocks manifesting diverse degrees of hydrothermal alteration. The study reveals intense chloritization of biotite in Fushou Mountain granite and feldspar altering into clay minerals, with the highest content reaching 30% to 40%, primarily consisting of kaolinite and montmorillonite. As the depth from the adit entrance increases, there is a discernible trend of diminishing weathering intensity in the granite. Moreover, the process of deep-seated rock alteration is primarily governed by the presence of structural planes, which subsequently gives rise to the development of alteration zones with a defined width on either side of these planes. The mechanical properties of highly weathered granite deteriorate significantly. With an increase in the depth of the adit, the mechanical properties of the rock exhibit a strengthening trend with reduced weathering intensity. However, noticeable reductions occur within the hydrothermal alteration zones. Silicification enhances the point load strength of rocks while simultaneously causing a considerable decrease in longitudinal wave velocity and rebound value. Field observation, scanning electron microscopy, and quantitative indices calculated by weight analysis were used to evaluate the alteration classification. The method used to evaluate the effect of alteration degree on Fushou Mountain granite provides a scientific basis for the construction of a pumped storage power plant.