The paper proposed a principle of interaction between plastic volume strains of and shear for rock and soil. The main meanings of the principle were further explained. Especially, the manners of the interaction between plastic volume strain and shear strain, the direct effect of plastic volume strain on the resistance to shear, the conditions of generating contraction and expansion of volume, the fact that the dependence of stress path is a comprehensive performance form of the interaction between the plastic volume strain and shear and its verification, and the fact that the critical state is a pure shearing process, in which the plastic and elastic volume strains remain constant, were demonstrated. In addition, according to this principle, it was theoretically proved that the space critical state line exists and is unique and independent of the stress history.