Abstract: Partial drainage tests on saturated sand are carried out by means of STDTTS standard stress path triaxial apparatus made by GDS Corporation of UK. By considering the aspects of excess pore water pressure, effective stress paths and asymptotic states, the mechanical properties of saturated sand under partial drainage are studied. By coupling asymptotic equation with dilatancy equation and introducing a status parameter named after ξ that describes the degrees of partial drainage, an equation for phase change stress ratio is presented. By introducing it into the constitutive model for soils considering stress paths, a constitutive model for saturated sand considering asymptotic states is established. The rationality of the model established is verified by the triaxial test results of saturated sand under partial drainage. The results indicate that drainage conditions control the expressing degrees of dilatancy of soils, and then affect the mechanical properties of soils from two aspects. One is the loading conditions that influence the effective stress paths of soils and the patterns of loading. The other is soil property conditions that influence the expressing degrees of the shear strength of soils, which lead to asymptotic states.