Seeking　a　facile　strategy　to　synthesize　efficient,　inexpensive　and　robust　flame　retardants　which　endow　the　polyurethane　(PU)　composites　with　desirable　flame　retardancy　and　mechanical　property　remains　compelling　but　challenging.　Here,　we　report　the　surface　modification　of　commercial　ammonium　polyphosphate　(APP)　by　3-amino-1,2-propanediol　in　water　solution　to　construct　a　novel　organic–inorganic　hybrid.　The　functionalized　APP　(FAPP)　with　multi-hydroxyl,　which　acts　as　both　intumescent　flame　retardants　and　crosslinking　agent,　was　covalently　introduced　into　solvent-free　PU　composite　via　a　thermo-curing　process.　Interestingly,　FAPP　exhibits　good　compatibility　and　strong　interfacial　interactions　within　PU　matrix,　which　in　turn　significantly　enhances　the　tensile　strength　(~40%)　of　the　resultant　PU　composite,　but　without　compromising　the　flexibility.　With　the　incorporation　of　15　wt%　FAPP,　the　PU　composite　reaches　a　limiting　oxygen　index　of　27.2%,　and　passes　UL-94　V-0　rating.　Meanwhile,　the　peak　heat　release　rate　and　total　heat　release　are　respectively　decreased　by　64.5%　and　37.8%　over　that　of　pristine　PU,　which　is　far　superior　to　original　APP.　The　high　flame-retardant　efficiency　primarily　results　from　the　formation　of　compact　and　intumescent　char　layers　in　condensed　phase.　Accordingly,　our　practice　may　be　extendable　to　the　implementation　in　other　high-performance　flame-retardant　polymer-matrix　composites.　©　2021　Elsevier　B.V.