Resolving　the　flammability　of　poly(L-lactic　acid)　(PLA)　while　ensuring　its　environmental　friendliness　and　pre-serving　key　flame　retardancy　and　mechanical　properties　represents　a　critical　challenge.　We　have　successfully　developed　a　highly　efficient　and　environmentally　friendly　flame　retardant　called　Hexamethylenediamine　tetra　-methylene　phosphonic　acid　amine　(HDME).　The　flame　retardancy　of　PLA/HDME　composites　was　significantly　improved,　as　indicated　by　the　LOI　value　of　29.1　%　and　UL-94　V-0　rating　for　PLA/3.5　HDME　with　only　3.5　%　HDME　addition.　The　results　show　a　23.4　%　reduction　in　the　total　heat　release　(THR),　a　40.0　%　increase　in　the　time　to　ignition　(TTI),　and　a　21.2　%　increase　in　the　flame　propagation　index　(FPI)　compared　to　original　PLA.　Flame　retardant　mechanism　of　HDME　involves　the　gas　phase,　condensed　phase,　and　interrupted　heat　exchange　effects.　The　HDME　also　preserved　the　original　mechanical　properties　of　PLA,　with　the　elongation　at　break　and　tensile　strength　retention　of　PLA/3.5　HDME　reaching　93.05　%　and　89.65　%.　This　work　provides　a　simple　and　efficient　method　for　flame　retardant　modification　of　PLA,　which　can　expand　its　application　scope.