Abstract: The principle of effective stress is the key stone of soil mechanics, which becomes the fundamental difference between soil mechanics and solid mechanics. As it is put forward, the principle of effective stress is always in the debate, and the discussion focus is its approximation. From the analysis of the assignment of the principle of effective stress, the connotation of the principle of effective stress is illustrated as the general rule between the measurable effects and external loads. The effective stress establishes a connecting bond on the external loads and the measurable effects such as strength or deformation, which is defined as the stress that can influence the measurable effects such as strength or deformation directly. Therefore, the effective stress is not unique, which is the nominal term of real stress and can be given according to the equilibrium of forces and the interaction of soil particles. The different formulas correspond to different definitions of the effective stress, which explains the approximation of Terzaghi principle of effective stress. Then, the physical meanings of the effective stress related to the strength and the deformation are given respectively, indicating the general law of effective stress-strength or effective stress-deformation. Based on the Mohr-Coulomb strength theory, the relationship between the effective stress related to strength and the shear strength is established.