Flexible, lightweight, and electrically insulated materials with high thermal conductivity and good mechanical performance are highly required for next-generation electronic devices. Hexagonal boron nitride nanosheet (BNNS) has a great potential to meet these requirements because of its high thermal conductivity, intrinsic insulation, and superior stability. However, the production of solution-processing, high quality BNNSs with larger lateral size is still challenged. Herein, we develop a thin-layer confined shearing and polymer-anchoring strategy for the exfoliation of hexagonal boron nitride (h-BN) into BNNSs. Upon optimization of the rotation speed of milling tools, chemical structure of anchored polymer, the layer thickness and rheological properties of liquid medium, the shearing force applied on h-BN crystals increases greatly. The anchored polymer (sodium carboxymethyl cellulose, CMC) can effectively promote shearing force transfer and impact buffer onto the h-BN crystals, consequently improving the exfoliation yield (84.6%) and giving BNNSs with large lateral size (1.97 ± 1.09 μm) and high aspect ratio (~ 746). The anchored polymer endows BNNSs with good hydrophilicity and solution processability. The BNNS (78 wt.%)/CMC film prepared by spatially confined evaporation has densely packed structure, exhibiting high tensile strength (289 ± 21 MPa) and high in-plane thermal conductivity (59.5 ± 2.7 W·m⁻¹·K⁻¹). Electrically-insulating yet highly thermally conducting nature enables BNNS films attractive in insulated thermal management applications.