The boron nitride nanosheet (BNNS) exhibits outstanding thermal conductivity, low dielectric properties, and high insulation characteristics, making it widely recognized in the industry as the next-generation material for thermal management. However, achieving low-cost scalable production of BNNS and assembling it into high thermal conductivity materials pose numerous challenges. Mechanical milling is an important method for the low-cost mass production of BNNS. Nevertheless, during the milling process, the impact force generated by ball collisions predominantly acts on the boron nitride, causing its fragmentation and resulting in prepared nanosheets with small dimensions and high thickness, significantly limiting the performance of assembled materials. Addressing this critical issue, an approach termed high-viscosity additive-assisted mechanical milling has been developed. By controlling the viscosity of the milling system, the impact force during milling is reduced by a factor of 10, while the shear force is increased by over 10 times. Using this method, the aspect ratio of BNNS produced is one to two orders of magnitude higher than that achieved by existing mechanical exfoliation methods. Employing produced BNNSs, a series of macroscopic material assembly techniques have been developed for the preparation of thermal management materials with high thermal conductivity and flexibility, among other characteristics.