In　order　to　estimate　accurately　the　warping　deformation　of　perforated　diaphragms　in　girder　distortion,　a　model　of　L-shaped　plate　is　proposed　in　this　paper　based　on　the　symmetry　for　the　structure　and　loading,　and　the　warping　behavior　under　concentrated　load　at　corner　is　investigated.　The　L-shaped　plate　is　seen　as　an　assembly　of　three　rectangular　portions　connected　by　undetermined　displacements　and　moments　along　the　common　boundary.　Analytical　functions　for　warping　displacements　of　each　rectangular　portion　are　constructed　according　to　elastic　plate　theory,　and　closed-form　solutions　are　derived　based　on　boundary　conditions　and　compatibility　between　portions.　Verifications　are　conducted　by　employing　Finite　Element　Method　and　agreeable　resultants　are　demonstrated　for　warping　displacements　and　in-plane　stress　components,　especially　for　the　stress　concentration　around　the　corner　of　the　cutout.　Besides,　a　cutout　ratio　rho　is　introduced　to　testify　the　feasibility　of　plate　theory　in　solving　the　warping　deformation　of　L-shaped　plate.　Results　show　that　the　rational　range　of　cutout　ratio　for　the　model　of　L-shaped　plate　is　identified　0.13　＜　rho　＜　0.25,　and　the　variation　of　the　shape　of　cutout　hardly　makes　any　significant　influence　on　the　warping　deformation　at　loading　corner　for　L-shaped　plate.