Determination of the complete stress–strain characteristics of rock is an essential issue in rock mechanics and plays an important role in solving excavation problems. To reflect the whole damaging process, a three-dimensional damage-softening statistical constitutive model for rock was established based on the Weibull distribution of mesoscopic element strength and by adopting the Generalized Zhang-Zhu (GZZ) criterion as the distribution parameter. The GZZ criterion is a three-dimensional Hoek–Brown strength criterion taking account of the influence of the intermediate principal stress. The results from the model were compared with existing experimental data, indicating that the model is feasible to describe the mechanical behavior of brittle rock. The influence of the Weibull distribution parameters on the constitutive model was studied and the results were generally consistent with physical interpretations of the Weibull parameters. A computer program for this constitutive model was developed, providing an easy way for simulating true triaxial tests under different conditions.