Under engineering conditions (i.e. the normal temperature and the middling or lower confining pressure), the microscopic mechanism of both plastic strain and damage variable in rock are the birth and the growth of the microfracture in a rock cell body. Thus the plasticity and damage shall appear at the same time and their evolution laws shall be coupled each other. For the above reasons, this paper established a theoretical frame of the constitutive model for elastoplastic damage in rock materials. In the frame the plastic strain rate is non-normal to the loading surface due to damage, but the obtained constitutive matrix is sysmmetrical. The non-normality is consistent with experimental data and the sysmmetry of the matrix is extremly important in the numerical analysis and theoretical research.