Considering the insufficiency of the present theory of rock damage model, this paper presents that the rock damage can be divided as damage materials and the rock microdefects caused by the materials damage on the basis of the study of the mechanism of rock deformation and the process of rock damage. Then a new definition of rock damage and impact factor γ between the damage materials and the microdefects is presented. Meanwhile, ascertain parameters, the basic properties of γ, and a rock damage model is established according to micro rock force analysis. Furthermore, a statistical constitutive model is established by introducing the statistical damage theory and lucubrating the relationship between effective stress and normal stress. The model well reflected the strain- softening characteristics of rock and strain- hardening of rock and had the definite physics meaning parameters. Therefore, it is easy to be applied. Finally, the model is proved to be rational and feasible by comparing with the existing research results and measured data.