The　structure　of　hexagonal　boron　nitride　(h-BN)　is　similar　to　that　of　graphite　and　its　practical　application　in　polymer　composites　is　blocked　by　incompatible　surface　nature.　Here,　we　presented　a　facile　method　to　simultaneously　exfoliate　and　organically　modify　h-BN　nanosheets　with　Lewis　acid-base　interaction.　Due　to　the　affinity　of　the　nitrogen　atoms,　the　synthesized　nucleophilic　flame　retardant　(Lewis　bases)　bonded　with　boron　atoms　of　h-BN　nanosheets　(Lewis　acids)　and　facilitated　the　direct　exfoliation　of　bulk　h-BN.　The　structure　and　morphology　of　flame　retardant　functionalized　BN　(f-BN)　nanosheets　were　well　characterized.　Then　performances　of　f-BN　nanosheets　as　advanced　nanofillers　in　TPU　composites　were　investigated　deeply.　As　confirmed　by　SEM,　XRD,　and　TEM,　f-BN　nanosheets　were　well-dispersed　in　the　TPU　matrix　and　formed　strong　interfacial　interaction　with　the　polymer　matrix.　The　dramatic　barrier　effect　of　f-BN　nanosheets　and　intrinsic　flame　retardant　of　synthesized　phosphonated　polyethyleneimine　(P-PEI)　significantly　improved　thermal　stability,　fire　safety,　and　smoke　suppression　of　TPU　composites.　By　adding　5.0　wt%　f-BN　nanosheets,　the　maximum　mass　loss　rate,　peak　heat　release　rate　(pHRR),　and　smoke　produce　rate　(SPR)　were　decreased　by　20.8　%,　68.0%,　and　53.6%,　respectively.　Moreover,　the　mechanical　properties　of　TPU　composites　were　enhanced.　The　self-stiffen　effect　of　TPU　composites　was　successfully　achieved:　after　108　k　dynamic　compressive　cycles,　the　storage　modulus　of　TPU/fBN-5.0　exhibits　a　13.8%　increase.　Based　on　Lewis　acid-base　interaction,　designing　a　nucleophilic　flame　retardant　to　simultaneously　exfoliate　and　functionally　modify　h-BN　nanosheets　offers　a　promising　avenue　for　the　preparation　of　functionalized　BN　nanosheets,　thus　broadening　its　practical　application　in　the　polymer　material　field.