Boron nitride nanosheets (BNNS) film receives wide attention in both academia and industry due to its high thermal conductivity (TC) and good electrical insulation capability. However, the brittleness and low strength of BNNS film largely limit its application. Herein, edge-functionalized BNNS (f-BNNS) with well-maintained in-plane crystalline structure was firstly prepared utilizing urea in the aqueous solution via ball-milling for the purpose of improving its stability in solvents such as water as well as maintaining their original high in-plane TC. Then, a biodegradable and highly thermally conductive film with an orderly oriented structure was prepared just by simple vacuum-assisted filtration via introducing small amount of cellulose nanofiber (CNF). The modification of BNNS and introduction of CNF results in a better orientation of f-BNNS, sufficient connection between f-BNNS themselves, and strong interaction between f-BNNS and CNF, which not only make the prepared composites film strong and tough but also possess higher in-plane TC. An increase of 70% in-plane TC, 63.2% of tensile strength and 77.8% of elongation could be achieved, respectively, for CNF/f-BNNS film, compared with that for CNF/BNNS film at the filler content of 70%. And it is interesting to find that the in-plane TC could be greatly enhanced as decrease of the thickness of film, and a value of 30.25 W/m K can be achieved at the thickness of ~ 30 μm for the film containing 70 wt% f-BNNS, whose in-plane TC can be comparable with that of neat BNNS film (51.5 W/m K). Although at such the high f-BNNS content, this composites film is even able to be bended and folded while 100% BNNS film cannot, indicating that this highly thermally conductive film could have potential applications in next-generation highly powerful and collapsible electronics devices.