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.