To investigate the damage mechanism of mechanical properties of granite under the action of acidic solution, the granite is saturated for 72 days in chemical solutions with pH values of 7, 5, and 3, respectively. The saturated specimens and natural drying specimens are then exposed to triaxial compression, nuclear magnetic resonance, scanning electron microscope, and Xray diffraction tests. Finally, the variation laws of triaxial compressive strength damage, deformation characteristics, and mechanical parameters of granite specimens are compared and analyzed in an acid solution. The results show the following: (1) The triaxial compressive strength, elastic modulus, and cohesion of granite specimens decrease with a decrease in pH value of an acid solution, whereas the Poisson′s ratio increases with the decrease of pH value. (2) The consumption of mineral components such as plagioclase and calcite in the specimens by acid solution corrosion increases the degree of internal structural porosity of the specimens, and microcracks are formed under the action of strong acid. (3) When compared with the specimens under natural drying conditions, the deviatoric stress peak of the specimens in acid solution acidification with a pH value of 3 decreases by 39.94% under confining pressure of 20 MPa. (4) As a result of the acid solution′s action, some minerals of granite dissolve and migrate, resulting in the continuous deterioration of pore structure and a large number of secondary pores. The higher the proportion of medium and large pores in an acid solution, the lower is the pH value.