Abstract: In this paper, the concept of absorbed suction between soil grains caused by capillary response is given and expounded , and the relation and basic distinction among this absorbed suction, surface tension and capillary pressure of the soil are established. The law of absorbed suction change and the primary factors affecting it are approached,and some problems in existing principle of effective stress for unsaturated soil analyzed. Based on this absorbed suction concept, the classification of saturation in soil and a principle of effective stress are proposed for unsaturated soils. The emperical parameter χ in Bishop′s effective stress formula is defined. Lastly, the primary factor affecting the effective stress or χ in unsaturated soils, and the principle for classification of unsaturated soils and its mechanical methods to analyze unsaturated soils are discussed.