The　nonlinear　unified　strength　theory　in　β　stress　space　is　similar　to　the　Drucker-Prager　strength　theory　in　principal　stress　space.　Compared　with　the　establishment　of　Cam-clay　model　with　stress　parameters　of　p　and　q,　a　new　three-dimensional　elastoplastic　constitutive　model　is　established　in　β　stress　space.　A　new　dilatancy　equation　is　proposed,　and　then　plastic　potential　function　is　obtained　combined　with　the　orthogonality　condition.　The　new　model　is　based　on　the　nonassociated　flow　rule　and　yield　function　of　modified　Cam-clay　model.　It　is　a　new　idea　to　establish　three-dimensional　elastoplastic　constitutive　model　directly.　The　proposed　model　in　which　the　failure　condition　is　based　on　nonlinear　unified　strength　theory　can　reflect　the　deformation　and　strength　characteristics　of　soils　reasonably,　and　can　be　devolved　into　the　Cam-clay　model.　©,　2015,　Chinese　Society　of　Civil　Engineering.　All　right　reserved.