Abstract: In this paper, the suction between grains of soils is comprehensively classified, and the properties of different suction is systematically expounded. Based on total suction and Mohr-Coulomb′s criteria of shear strength,the principle of generalized effective stress and formula of generalized shear strength are proposed.Each component of the generalized shear strength is analyzed. By generalized shear strength theory, the Bishop′s and Fredlund′s formula of shear strength are unified formally, and the physical meanings of the state variables in the formula of generalized shear strength are clear and definite.The primary factors affecting the generalized effective stress and the generalized shear strength are discussed. The experiments on nonlinear direct ratio and curred relation between shear strength and matric suction of clayey and sandy soil respectively, which were done by Donald, Gan, Escario, Fredlund, etc.,are cited to prove or to be explained by the generalized shear strength theory. This new theory based on total suction can objectively depict the shear strength of unsaturated clayey or sandy soil.