Abstract: By analyzing some misunderstandings about the principle of effective stress for saturated soil, the physical meaning of effective stress of soil is given interaction between soil based on the particle and water. The effective stress of soil is the effective load transferred between the soil particle skeletons in a unit section and equal to the total stress minus the pore water pressure. By using the mechanics balance equation of soil, on the one hand, the Terzaghi principle of effective stress for saturated soil is proved and its sphere of application is analyzed, on the other hand, the Bishop principle of effective stress for unsaturated soil is proved and the physical meaning of parameter χis explained. By combining the two principles in the same mechanics frame, it is illustrated that the Terzaghi principle of effective stress for saturated soil is one special case of Bishop principle of effective stress when the saturation level χ=1 in theory, and the sphere of application of the Bishop principle of effective stress for unsaturated soil is also analyzed. The researches show that by analyzing the principle of effective stress from two aspects of loading and soil properties, the physical meaning of effective stress is explained clearly, and the mechanism of the influence of pore water pressure mechanics characters of soil is also proposed.