This　paper　presents　a　direct　method　to　establish　a　3D　elastoplastic　constitutive　model　for　soils　based　on　the　characteristic　stress.　In　the　established　model,　a　new　yield　function　is　first　formulated　through　an　interpolation　function　in　the　characteristic　stress　space.　A　hardening　parameter　that　suits　for　monotonic　and　cyclic　loading　conditions　is　established　by　introducing　the　concept　of　the　subloading　surface.　Then,　a　3D　constitutive　model　for　soils　is　established　by　combining　the　previously　proposed　plastic　potential　function.　Only　seven　material　parameters　with　clear　physical　meanings　are　included　in　the　proposed　3D　elastoplastic　model.　The　deformation　behaviours　of　soils　under　monotonic　and　cyclic　loading　conditions　and　the　critical　state　properties　under　true　triaxial　stress　conditions　are　analysed　on　the　basis　of　the　presented　model.　The　capability　of　the　model　is　verified　by　the　comparisons　between　the　model　responses　and　the　test　results　of　soils　under　the　3D　monotonic　and　cyclic　loading　conditions.