The　constitutive　model　in　geotechnical　engineering　plays　a　critical　role,　driving　the　need　for　ongoing　research　and　improvement.　In　this　study,　by　utilizing　a　single　yield　curve　and　only　one　hardening　parameter,　a　nonorthogonal　elastoplastic　(NOEP)　constitutive　model　for　the　overconsolidated　(OC)　clay　is　established　within　the　framework　of　the　NOEP　model.　The　nonorthogonal　plastic　flow　rule,　based　on　the　Riemann-Liouville　fractional　derivative,　was　employed　to　determine　the　plastic　flow　direction,　which　is　nonorthogonal　to　the　improved　yield　curve.　Furthermore,　a　novel　hardening　parameter　is　proposed　to　capture　the　magnitude　of　plastic　strain　increment　for　OC　clays.　This　is　achieved　by　comparing　deformation　behaviors　in　OC　clays　with　two　distinct　degrees　of　overconsolidation.　Additionally,　it　is　further　enhanced　by　the　integration　of　a　newly　proposed　potential　stress　ratio.　The　proposed　model　requires　only　eight　parameters,　and　its　performance　is　evaluated　by　its　predictions　with　test　results　for　OC　clays　subjected　to　drained　or　undrained　triaxial　stress　conditions.　This　study　is　expected　to　offer　valuable　insights　and　a　better　approach　toward　understanding　and　capturing　the　deformation　behaviors　of　OC　clays.