Plate　stiffeners　are　the　basic　components　of　steel　boxes,　steel　towers,　and　steel　arches　in　steel　structural　bridges.　Local　instability　of　plate　stiffeners　is　the　primary　mode　of　failure.　To　study　the　local　stability　performance　of　plate　stiffeners,　two　groups　of　plate-stiffener　local-stability　specimens　with　different　thicknesses　and　widths　were　designed　for　an　axis　compression　test,　and　corresponding　finite　element　models　were　established.　The　effects　of　the　constitutive　relationship,　residual　stress,　and　local　initial　geometric　imperfections　on　the　local　stability　performance　were　analyzed.　Simplified　calculation　formulas　for　the　local　stability　of　plate　stiffeners　and　a　trilateral　simply　supported　plate　were　obtained.　Experimental　and　analytical　results　show　the　following.　(1)　When　the　width-to-thickness　ratio　of　the　plate　stiffener　is　less　than　16,　simultaneous　buckling　failure　occurs　in　the　plate　stiffener　and　stiffened　plate,　on　the　contrary,　local　buckling　failure　occurs　in　the　plate　stiffener.　(2)　With　an　increase　in　the　width-to-thickness　ratio　of　the　plate　stiffener,　instability　deformation　of　the　specimen　becomes　increasingly　obvious.　For　the　specimen　with　variable　stiffener　thickness,　the　ultimate　average　stress　first　increases　and　then　decreases　with　an　increase　in　the　width-to-thickness　ratio　of　the　stiffener.　For　the　specimen　with　a　variable　stiffener　width,　the　ultimate　average　stress　decreases　gradually　with　an　increase　in　the　width-to-thickness　ratio.　(3)　When　the　relative　width-to-thickness　ratio　exceeds　0.91,　the　fitted　cubic　polynomial　fitting　curve　of　the　plate　stiffener　in　the　stiffened　plate　member　is　higher　than　those　of　other　code　curves.　In　addition,　when　the　relative　width-to-thickness　ratio　is　less　than　0.95,　the　fitted　formula　curve　of　the　trilateral　simply　supported　plate　approximates　those　of　the　GB　50017-2017,　Eurocode　3,　and　American　AISI　codes.　The　entire　range　of　relative　width-to-thickness　ratios　is　found　to　be　higher　than　the　Chinese　and　Japanese　steel　bridge　codes.　(4)　The　fitting　formula　of　the　plate　stiffener　in　the　member　can　better　calculate　the　bearing　capacity　of　the　actual　specimen,　and　the　fitting　formula　of　the　trilateral　simply　supported　plate　is　safer.　Using　the　fitting　formula　of　trilateral　simply　supported　plates　for　calculation　is　thus　recommended.　©　2022　Xi＇an　Highway　University.　All　rights　reserved.