In view of the mechanical properties of coarse-grained materials, such as strain softening and dilatancy, a generalized shear damage mechanical model with wide applicability was established in this study by considering the strain localization phenomenon marked by shear band. This damage model adopted the mathematical simplification of shear band in the envelope theory, and the stress-strain relationship equation of coarse-grained material was derived based on the strain equivalence principle and Weibull distribution. A nonlinear functional relationship between axial and volumetric plastic strain was proposed to describe the weakening of dilatancy based on the mechanism of dilatancy. Combined with the servo process of coarse-grained materials in triaxial compression tests, a method to determine the parameters of damage model was proposed based on genetic algorithm. By conducting a series of triaxial compression tests under different confining pressures, the shear damage mechanical model was validated, and the effects of the evolution of shear band parameters on the strength and deformation characteristics of coarse-grained materials were further analyzed. The results indicate that the proposed shear damage mechanical model considering the strain localization characteristics can accurately simulate the strain-softening and dilatancy characteristics of coarse-grained materials, and effectively reveal the influence mechanism of the internal deformation of the shear band on the overall macroscopic deformation of the coarse-grained sample. The evolution of the shear band parameters with the surrounding confining pressures in the model was consistent with the mesoscopic mechanism of coarse-grained materials. The strength composition calculated by this model was in good agreement with the micro mechanism, such as the breakage and reorganization of coarse-grained particles.