Abstract: The strength theory is an important research subject in advanced mechanics. Many strength theories and important experimental results have been put forward by the scholars all over the world since 200 years ago. Different mathematical expressions were deduced based on different assumptions and mechanical models in these theories which can only be applied to some specific materials. What is the relationship among various strength theories? Can we propose a unified strength theory that adapted to more kinds of materials? Considerable effort has been devoted to this important problem by many scientists all over the world since the end of the 19th century, but it has not been solved. The three-shear stress unified strength theory has been put forward and verified by the method of combining the theoretical study with the experimental verification in this paper. It is considered that the material will failure when the function of the three principal shear stresses and their corresponding three normal stresses which acting on the main shear planes of the dodecahedron element reaches a magnitude. The three-shear stress unified strength theory is the whole force theory, it uses a unified linear expression to contain or approximate the existing and other newly various single and unified, linear and unlinear, convex and non-convex strength theory, it forms a new strength theory system which the inner boundary is the single-shear strength theory and its external boundary is the three-shear stress strength theory, it brings about the highly unity of the strength theory, it develops the unified strength theory which can suitable for different kinds of materials under various stress states from the single-shear strength which only suitable for some specific materials under specific stress states, and it can make full use of the potential strength of the materials. It is shown that the three-shear stress unified strength theory can be widely suitable for different kinds of materials under various stress states from the fact that a large number of experimental results of materials are in agreement with this theory.