A damage constitutive model was established to evaluate the stability of jointed rock masses around the underground caverns of Shuangjiangkou Hydropower Station. In this model, a second-order damage tensor was formulated to describe the initial geometric imperfection of jointed rock masses. A failure criterion for crack coalescence and a damage evolution equation were proposed by considering the secondary cracks induced by excavations according to the fracture mechanics principles. A subprogram under the framework of FLAC-3D was developed for the damage constitutive model and the damage evolution equation. The model and program were then applied in the stability assessment for the Shuangjiangkou underground cavern group.