Seeking efficient thermal management materials for electronic components is urgent and important. BN has high thermal conductivity and good insulating properties, making it an ideal polymer filler for improving thermal conductivity. Currently, because of the weak interaction between BN and its high melting point, it is extremely difficult to construct a BN three-dimensional skeleton by direct sintering to achieve efficient heat transfer channels. In this work, vertically aligned BN three-dimensional skeletons with fiber walls were synthesized in one step by combining the in-situ conversion of precursor fibers and directional freeze-drying technology. The precursor fibers were expelled by the directionally grown ice crystals, forming vertically aligned fiber walls. Importantly, molten B₂O₃ produced during air calcination made the fibers bond to each other, thus obtaining the vertically aligned and interconnected BN fiber skeleton. Benefiting from the short-path fast heat transfer and low interfacial thermal resistance of the fiber walls, the obtained vertically aligned BN skeleton exhibited obvious structural advantages, resulting in composites with a unidirectional thermal conductivity of 1.372 W/(m·K) with the filling content of 25.7 wt% and high enhancement efficiency per unit volume of 32.6 %. This approach is expected to open a new avenue for developing highly thermally conductive composites with great potential.