<p>Altered rock, often encountered in major engineering projects, can seriously affect the stability of rocks and slopes surrounding deeply buried tunnels. This study addressed the alteration mechanism, alteration degree classification, and mechanical parameters of altered rock in engineering project areas using field testing, thin slice identification, X-ray diffraction, and major element testing. Results showed that the altered rock types in the areas of the Pingjiang Pumped Storage Power Station, Hunan Province, and a diversion tunnel in northern Xinjiang Uygur Autonomous Region include biotite granodiorite, biotite monzogranite, and cataclastic granite, and that the main alteration mechanisms are chloritization of biotite and clayization of feldspar minerals. The altered rocks were classified as slightly, moderately, or strongly altered according to their apparent characteristics, rebound value, longitudinal wave velocity, metamorphic mineral content, and porosity. The bulk density, elastic modulus, cohesion, and internal friction angle (Poisson’s ratio) of the altered rocks decreased (increased) with increase in the degree of alteration. Numerical simulations showed that in altered rock slope areas, the zone of strong rock alteration and the moderate-strong alteration contact zone exhibit locally large deformations that represent a certain hazard to engineering projects. These results provide valuable guidance and support for major projects in altered rock areas.</p>