A　non-orthogonal　elastoplastic　model　for　clay　is　proposed　by　combining　the　non-orthogonal　plastic　flow　rule　with　the　critical　state　concept,　and　the　model　framework　is　presented　from　the　perspective　of　the　magnitude　and　direction　of　the　plastic　strain　increment.　The　magnitude　is　obtained　based　on　the　improved　elliptical　yield　function　and　the　plastic　volumetric　strain　dependent　hardening　parameter.　The　direction　is　determined　by　applying　the　non-orthogonal　plastic　flow　rule　with　the　Riemann-Liouville　fractional　derivative　to　the　yield　function　without　the　necessity　of　additional　plastic　potential　function.　The　presented　approach　gives　rise　to　a　simple　model　for　soil　with　five　parameters.　All　parameters　have　clear　physical　meaning　and　can　be　easily　identified　by　triaxial　tests.　The　model　performance　is　shown　by　analyzing　the　evolution　process　of　the　yield　surface,　the　hardening　rule　and　the　plastic　flow　direction.　The　capability　of　the　proposed　model　to　capture　the　mechanical　behaviours　of　clay　with　different　stiffness　is　also　confirmed　by　predicting　test　results　from　the　literature.