Flexible polymer nanocomposites reinforced by high-dielectric-constant ceramic nanofillers have shown great potential for dielectric energy storage applications in advanced electronic and electrical systems. However, it remains a challenge to improve their energy density and energy efficiency at high temperatures above 150°C. Here, we report a nanofiber-reinforced polyetherimide nanocomposite employing BN-BaTiO 3 heterogeneous nanofibers as fillers, where the BN nanoparticles were embedded inside BaTiO 3 nanofibers to create BN/BaTiO 3 /PEI hierarchical interfaces. The high dielectric constant and the geometric large aspect ratio of the BN-BaTiO 3 heterogeneous nanofibers lead to simultaneously increased dielectric constant and breakdown strength of the nanocomposite over a broad temperature range. In particular, the emerging hierarchical BN/BaTiO 3 /PEI interfaces enable promoted density and energy level of traps for the mobile charges, which further suppresses the conduction loss and improves the breakdown strength under high temperatures, as confirmed by a combination of thermally stimulated depolarization current measurement and phase-field simulation.Finally, the nanocomposite with hierarchical interfaces boosts an ultrahigh energy density of 5.23 J cm −3 with an energy efficiency of > 90% at 150°C, which is the highest energy density reported so far in nanofiber-reinforced polymer nanocomposites and also outperforms most nanocomposites counterparts dispersed with nanoparticles and nanosheets. Our work demonstrates hierarchical interface engineering as an effective strategy to promote the high-temperature energy storage performance of fiber-reinforced polymer nanocomposites, which is of significance for their applications in high-temperature harsh conditions.