Abstract: The research on the plasticity mechanism and constitutive equation of materials is an important area. Although many works have been done in this area, unfortunately, it is difficult to guide the exploitation, processing and development of materials qualitatively and quantitatively in theory up to now. The thermodynamic micromechanism of the plasticity for crystal materials, and concrete researches on the plasticity and its constitutive relationship for metallic single crystal materials are proposed. Furthermore, some important quantitative relationship equations having not appeared before about the plasticity of crystal materials are deduced and obtained. They not only can explain and unify the various mechanism having been proposed, but also agree with the practical phenomena and problems.