Hexagonal boron nitride nanosheets (h-BNNSs) are known to exhibit numerous prominent performances. However, scalable synthesis of h-BNNSs with a green, simple method has been difficult to achieve. In this study, xylitol-assisted discontinuous ball milling exfoliation (XADBME) of hexagonal boron nitride (h-BN) was proposed as a novel strategy for scalable synthesis of h-BNNSs. First, the high yield (above 80 wt%) could be achieved smoothly with this green chemistry method. Moreover, the as-synthesized h-BNNSs exhibited large lateral size, ultra-thin, defect-free. Lastly, the as-synthesized BNNSs and cellulose nanofibers (CNFs) were compounded to form a series of membrane materials. The mentioned membranes exhibited high in-plane thermal conductivity and prominent UV absorption characteristics. 70 wt% BNNS membrane [3.4 W(mK)(-1)] exhibited nearly 400% higher in-plane thermal conductivity than that of membrane without BNNS [0.7 W(mK)(-1)]. Thus, it shows promising applications in thermal management of electronic display screens, eye protection equipment, as well as portable mobile devices.