This　study　investigates　the　modification　of　aluminum　hydroxide　(ATH)　powder　using　waterborne　polyurethane　(WPU)　as　a　novel　modifier,　along　with　its　subsequent　effects　on　the　dispersion,　mechanical　properties,　and　thermal　performance　of　ATH-filled　low-density　polyethylene　(LDPE)　composites.　ATH　was　modified　through　an　optimized　wet　process,　and　the　modification　efficiency　was　evaluated　using　various　characterization　techniques,　including　scanning　electron　microscopy　(SEM),　X-ray　diffraction　(XRD),　Fourier-transform　infrared　spectroscopy　(FTIR),　X-ray　photoelectron　spectroscopy　(XPS),　and　thermogravimetric　analysis　(TGA).　The　results　show　that　WPU,　as　a　modifier,　effectively　improved　the　dispersion　of　ATH　in　the　organic　phase,　as　demonstrated　by　the　reduced　settling　time　and　enhanced　interfacial　compatibility　between　ATH　and　LDPE.　The　modified　ATH　demonstrated　enhanced　mechanical　properties　in　LDPE-based　composites,　with　a　tensile　strength　of　30.02　MPa,　flexural　strength　of　13.20　MPa,　impact　strength　of　65.75　kJ/m2,　and　elongation　at　break　of　59.84%,　all　reaching　their　maximum　at　3.0　wt.%　WPU　modification.　Additionally,　the　flame　retardancy　of　the　composites　was　significantly　improved　due　to　the　incorporation　of　ATH,　with　the　ATH　content　in　the　composites　reaching　up　to　60%,　further　enhancing　their　fire　resistance.　This　study　highlights　the　effectiveness　of　WPU-modified　ATH　as　both　a　flame　retardant　and　a　reinforcing　filler　for　LDPE　composites,　offering　potential　advantages　in　enhancing　material　properties　while　reducing　manufacturing　costs.　©　2025　by　the　authors.