The　fractional　order　flow　rule　derived　from　the　fractional　calculus　could　be　used　to　describe　the　elastoplastic　deformation　of　soils　under　specific　stress　conditions.　However,　the　strength　and　deformation　properties　of　soils　under　the　true　triaxial　conditions,　which　are　extremely　different,　cannot　be　reflected　by　the　existing　fractional　models.　This　study　aims　to　propose　an　approach　to　develop　a　fractional　order　elastoplastic　constitutive　model　suitable　for　the　3D　stress　conditions.　The　development　of　the　new　model　refers　to　the　establishment　of　the　Cam-clay　model,　and　the　plastic　flow　direction　is　expressed　by　the　fractional　order　flow　rule　which　is　established　in　the　characteristic　stress　space.　Consequently,　the　concept　of　the　critical　state　is　also　introduced　into　this　developed　fractional　order　model.　Comparisons　between　the　test　data　and　the　predictions　show　that,　the　developed　model　can　directly　and　reasonably　reflect　the　strength　and　deformation　properties　of　clay　under　the　true　triaxial　conditions.　©　2018　China　Communications　Press　Co.　Ltd.　All　rights　reserved.