High heat-dissipation polymers are currently in great demand especially with the rapid development of electronic devices. However, traditional polymer composites usually suffer from both low thermal conductivity due to poor dispersibility and low concentration of fillers in the polymer matrix. To address this issue, it is necessary to improve the compatibility between the thermal conductive fillers and polymer matrix. Here, we designed a highly water-soluble functionalized boron nitride (FBN) nanosheet. Unlike most functional BN nanosheets that are only dispersible in water with polymer matrix at low concentrations, our FBN nanosheets can be mutually dispersed with aqueous polymers such as polyvinyl alcohol (PVA) in arbitrary weight ratios. The super compatibility between FBN and PVA is further interpreted by the Pickering emulsion formed from water and n-hexane. Moreover, after facile vacuum filtration, the robust FBN/polymer freestanding films with layer-by-layer laminate nanostructures are well fabricated. The nanocomposite films exhibit superior in-plane thermal conductivity (120 W m -1 K -1 for 90 wt% FBN loading in FBN/PVA film), which is nearly 100 times larger than that of the pristine PVA film. The FBN/polymer films provide good fire-retardant ability, thus effectively retarding flammability. In addition, the nanocomposite film with high concentration of FBN up to 70 wt% still possessed excellent flexibility and toughness even after being rolled and folded 100 times. Interestingly, the rolled film hollow cylinder supported 25:000 times its own weight without cracking, highlighting the extra strong interaction between the FBN and PVA. These properties make the nanosheet an excellent candidate for thermal management in electronics.