The　interpolation　function　method　was　employed　to　establish　the　Cam-clay　Model　and　Modified　Cam-clay　Model　for　normally　consolidated　clay.　Through　the　initial　pressure　density　states　(the　degree　of　underconsolidation),　the　isotropy　compression　curve＇s　ramp　rates　of　underconsolidated　soil　can　be　determined,　on　the　base　of　which,　an　elastoplastic　constitutive　model　for　underconsolidated　clay　was　proposed　by　employing　the　interpolation　function　method　in　this　paper.　The　model　established　has　the　same　material　parameters　as　the　Cam-clay　Model,　and　when　the　initial　pressure　density　states　is　on　the　normal　consolidation　line,　it　can　be　simplified　into　the　Cam-clay　Model.　The　authors　analyzed　the　function　of　model　established　by　the　consolidated　drainage　and　consolidated　undrainaged　tests,　and　the　model　can　reflect　rationally　the　mechanical　properties　of　underconsolidated　clay.