A　novel　three-dimensional　(3D)　fractional　plastic　flow　rule　that　is　not　limited　by　the　coordinate　basis　of　the　differentiable　function　is　proposed　based　on　the　fractional　derivative　and　the　covariant　transformation.　By　introducing　the　3D　fractional　plastic　flow　rule　into　the　characteristic　stress　space,　a　3D　fractional　elastoplastic　model　for　soil　is　established　for　the　first　time.　Only　five　material　parameters　with　clear　physical　significance　are　required　in　the　proposed　model.　The　capability　of　the　model　in　capturing　the　strength　and　deformation　behaviour　of　soils　under　true　3D　stress　conditions　is　verified　by　comparing　model　predictions　with　test　results.