In this paper, a novel method based on the liquid nitrogen-driven rotation and ice-templated assembly was proposed to fabricate a new kind of boron nitride nanosheet (BNNS) and BNNS-Ag spongy miscrosphere used as thermally conductive fillers. The liquid nitrogen driven assembly ultimately led to hierarchical 3D BNNS frameworks with radial alignments, forming a sea urchin-like microstructure. BN sphere/epoxy resin composites were finally obtained by infiltrating the as-prepared spongy microsphere with epoxy resin followed by thermal curing. At the sphere content of 2.7 vol%, the through-plane thermal conductivity of BNNS sphere/epoxy resin composite reaches 0.57 W/(m·K), while the value for BNNSAg sphere/epoxy resin composite reaches 0.64 W/(m·K), indicating the corresponding enhancement of 276.5% towards pure epoxy resin. The obtained composites exhibit strong potential for thermal management applications for a variety of technological needs, particularly electronic packaging. The combination of liquid nitrogen-driven rotation and ice-templated assembly was demonstrated to a useful tool to fabricate efficient fillers for thermal management applications.