The rapid miniaturization and high integration of modern electronic devices have brought an increasing demand for polymer-based thermal management materials with higher thermal conductivity. Boron nitride nanosheets (BNNs) have been widely used as thermally conductive fillers benefiting from the extremely high intrinsic thermal conductivity. However, the small lateral size and weak interface bonding of BNNs enabled them to only form thermally conductive networks through physical overlap, resulting in high interfacial thermal resistance. To address this issue, an innovative strategy based on interface engineering was proposed in this study. High-aspect-ratio boron nitride belts (BNbs) were successfully synthesized by carbon thermal reduction nitridation method through the in-situ generation and sintering of BNNs. The surface of BNb showed the sintering of numerous smaller-sized BNNs, which precisely addresses the issue of weak interfacial bonding between BNNs. On this basis, the as-synthesized BNbs were combined with nano-fibrillated cellulose (NFC) to prepare NFC/BNb composite films through a facile vacuum filtration process. Due to the thermally conductive network formed by the horizontal oriented arrangement of BNb and their particular morphological advantages, the NFC/BNb films demonstrated significantly higher in-plane thermal conductivity than that of NFC/BNNs films, achieving the highest value of 19.119 W·m⁻¹·K⁻¹ at a 20 wt% filling fraction. In addition, the NFC/BNb films also exhibited superior thermal stability, mechanical strength, flexibility, and electrical insulation performance, suggesting the significant application potential of the designed BNb fillers in the thermal management field.