Plate　stiffener　is　the　most　basic　form　of　open　stiffenerand　its　local　instability　is　the　main　failure　mode.　In　order　to　study　the　local　stability　of　plate　stiffenertwo　groups　of　Q420　open　flat　plate　stiffened　panel　speci　mens　with　different　thicknesses　and　heights　were　designed　for　axial　compression　testsand　the　corresponding　finite　element　model　was　established.　The　simplified　calculation　formulas　for　the　local　stability　of　the　plate　stiffener　and　tri　lateral　simply　supported　plate　were　obtained.　The　experimental　and　analytical　results　show　thatwhen　the　width-to-thickness　ratio　of　plate　stiffener　is　less　than　16the　simultaneous　buckling　failure　of　plate　stiffener　and　stiffened　plate　occurson　the　contrarythe　local　buckling　failure　of　plate　stiffener　occursWith　the　increase　of　the　width-to-thickness　ratio　of　plate　stiffenerthe　ultimate　average　stress　decreases　graduallyand　the　instability　deformation　of　the　specimen　becomes　more　obvious.When　the　relative　width-to-thickness　ratio　exceeds　1.08the　fitted　cubic　poly　nomial　fitting　curve　of　the　plate　stiffener　in　the　member　is　higher　than　that　of　other　code　curvesWhen　the　relative　width-to-thickness　ratio　exceeds　1.32the　fitted　cubic　polynomial　fitting　curve　of　trilateral　simply　supported　plate　is　higher　than　that　of　Euler　curveand　it　is　higher　than　the　curves　in　Chinese　steel　bridge　code　and　Japanese　code　in　the　whole　range　of　relative　width-to-thickness　ratio.　The　fitting　formula　of　plate　stiffener　in　the　member　can　accu　rately　calculate　the　bearing　capacity　of　the　actual　specimenand　the　fitting　formula　of　a　trilateral　simply　supported　plate　is　safer.　It　is　recommended　to　use　the　fitting　formula　of　a　trilateral　simply　supported　plate　for　design.　©　2023　Hunan　University.　All　rights　reserved.