In order to accurately and conveniently determine the mechanical behavior of hard rock in deep engineering structures, the following works have been conducted in this paper: according to the similar mechanism of damage and plastic, determination scheme of initial yield is assumed to be used for the damage threshold. Unified energy yield criterion of rock is adopted as the decision criterion of damage threshold. Based on the definition of damage variable by Rabotnov and hypothesis of Lemaitre strain equivalence, a simplified damage constitutive law is formulated by assuming the stress- strain relation of undamaged portion to conform Hooke’s law. Aimed at expressing brittle-ductile transition of hard rock under the condition from low confining pressure to high confining pressure, a modified Mazars damage evolution equation is proposed. Damage model parameters are determined based on conventional triaxial compressive test of T2b marble. The proposed model is contrasted with the model based on Mazars damage evolution and damage threshold criterion used by Mohr-Coulomb criterion respectively. It is suggested that the damage model can express brittle-ductile transition and simulate damage evolution after damage threshold in an accurate way, which has some reference for engineering calculation of brittle hard rock.