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.