The　strength　criterion　in　the　characteristic　stress　space,　whose　form　is　the　same　as　that　of　the　Drucker-Prager　strength　criterion,　can　describe　the　true　three-dimensional　strength　of　geomaterials.　In　other　words,　the　isotropic　function　in　the　characteristic　stress　space　can　describe　the　anisotropic　mechanical　properties　of　the　geomaterials.　In　the　characteristic　stress　space,　a　new　yield　function　is　proposed　for　the　normally　consolidated　soils　on　the　basis　of　the　interpolation　function　method.　By　using　the　plastic　potential　function　from　the　authors＇　previous　researches,　a　true　3D　elastoplastic　constitutive　model　for　normally　consolidated　soils　is　directly　established　in　the　characteristic　stress　space.　There　are　only　7　parameters　in　the　proposed　model.　All　the　parameters　have　clear　physical　meanings　and　can　be　easily　determined　through　laboratory　tests.　The　proposed　model　is　analyzed　through　model　simulations　and　is　verified　using　the　soil　data　available　in　the　literatures.　These　results　demonstrate　that　the　proposed　model　can　simply　and　reasonably　describe　the　characteristics　of　the　true　three-dimensional　strength　and　deformation　of　normally　consolidated　soils　and　can　be　degenerated　to　the　modified　Cam-clay　model.　©　2019,　Editorial　Office　of　Chinese　Journal　of　Geotechnical　Engineering.　All　right　reserved.