Polymer-based nanocomposite with high-dielectric-constant nanofillers has attracted lots of attention in dielectric material for energy storage as film capacitor. However, it is still challenging that the addition of stateof-the-art fillers always leads to electric field distortion and high leakage current resulted from poor insulation and breakdown strength of these fillers. In this study, to address this issue, a novel hybrid nanofiber with hierarchical structure was elaborately designed, which exhibits superior inherent dielectric property than conventional high-dielectric-constant fillers. The hierarchical hybrid nanofiber was fabricated by embedding boron nitride nanosheet (BNNS) into grain boundary interspace of barium titanate nanofiber to strengthen nanofiller straightforward, based on the bold conjecture that BNNS acted as solid barrier for electrical tree propagation and leakage current conduction inside nanofiller. Using hybrid nanofiber as filler, resulting nanocomposite exhibited simultaneously enhanced dielectric constant and breakdown strength and a much larger energy storage density (U-e approximate to 15.25 J cm(-3)) than conventional nanofiller-incorporated composite, which was 3.07 times of pristine matrix. The hierarchically designed hybrid nanofiber guaranteed good insulation and strong polarization of high-dielectric-constant nanofiller simultaneously and this work provides a novel and effective approach to improve the dielectric performance of composite dielectric.