The　significant　influence　of　transverse　isotropy　on　mechanical　behaviours　of　soil　has　long　been　recognised,　but　it　is　still　difficult　to　capture　well　under　3D　stress　conditions.　In　this　study,　a　method　of　establishing　3D　non-orthogonal　elastoplastic　(NOEP)　constitutive　models　for　transversely　isotropic　soil　(TI-soil)　is　proposed.　The　effect　of　transverse　isotropy　is　considered　by　extending　the　characteristic　stress　with　the　fabric　tensor　of　TI-soil.　The　extended　characteristic　stress　can　be　used　to　treat　the　mechanical　behaviour　of　TI-soil　from　the　perspective　of　isotropic　soil.　Therefore,　a　unified　strength　criterion　for　TI-soil　is　developed　in　the　same　form　as　the　Drucker-Prager　criterion.　In　order　to　reflect　the　non-orthogonality　of　the　plastic　flow　direction　with　respect　to　the　yield　surface,　the　non-orthogonal　plastic　flow　rule　is　adopted,　which　obviates　the　tedious　work　of　constructing　the　plastic　potential　function　or　the　dilatancy　relation.　Based　on　the　yield　function　and　hardening　parameter　expressed　in　the　extended　characteristic　stress　space,　specific　formulations　of　the　NOEP　model　for　TI-soil　can　be　derived.　Comparisons　between　model　predictions　and　test　results　show　that　the　developed　model　can　well　reflect　the　effects　of　transverse　isotropy　and　intermediate　principal　stress　on　the　strength　and　deformation　behaviour　of　TI-soil.