Abstract: The flexible heat-diffusion film composited from boron nitride nanosheet (BNNS) and polymer is one important kind of thermal management materials. To reveal the influence of polymer aqueous solution on the preparation of BNNS by homogenization exfoliation of hexagonal boron nitride (h-BN), and to explore the thermal conduction property and mechanism of the composite film prepared by BNNS and polymer, h-BN was dispersed into aqueous solutions of hydroxyethyl cellulose (HEC) or sodium carboxymethyl cellulose (CMC) and then subjected to micro-fluidization homogenization and exfoliation in this article. The prepared BNNSs were respectively composited with aramid nanofibers (ANF) or bacterial cellulose (BC) to form heat dissipation films. The effects of pressure, kind and concentration of the peeling medium on the size, morphology and structure of the BNNS were investigated, as well as the influences of the BNNS, ANF, and BC on the thermal conduction and mechanical properties of the composite films. The results indicated that the suitable viscosity of the polymer solutions can effectively coat the h-BN, which can not only prevent the re-aggregation of the h-BN and exfoliated BNNSs, but also buffer the impact force of the cavity on h-BN during the homogenization process, thereby contributing to obtaining BNNS with larger lateral sizes. The thermal conductivity (TC) of the BNNS/ANF composite film is superior to that of BNNS/BC due to the higher stacking density of BNNS/ANF and the higher intrinsic TC of ANF, and the highest TC value of the composite film has reached up to 20.22 W·m⁻¹·K⁻¹.