<p>The development of polymer dielectric nanocomposites with high energy density is key to promoting the miniaturization of film capacitors. However, the poor compatibility and dielectric mismatch between the nanofillers and the polymer matrix still cannot be well settled simultaneously at present. Herein, a novel interfacial engineering strategy using the covalent organic framework (COF) material is firstly employed to concomitantly tackle the above issues and achieve improved energy storage performance for the polymer-based dielectric nanocomposites. Specifically, the poly(vinylidene fluoride-co-hexafluoro propylene)/poly(methyl methacrylate) (P(VDF-HFP)/PMMA)–based nanocomposite with an ultra-low content of 0.5 wt% core–shell structured BaTiO₃@COF nanofillers exhibits an enhanced breakdown strength of 766.5 MV/m, yielding a high discharged energy density of 26.1 J/cm³, which outperforms the energy storage performance of most current PVDF-based/PMMA binary blend polymer composite dielectrics. More intriguingly, abundant experimental and theoretical evidence comprehensively demonstrates that the interfacial COF shell not only enhances the dielectric response but also improves the breakdown strength of the nanocomposites due to its higher electron affinity, which can act as the robust electron trap. Taking a step further, the dielectric capacitor based on the above nanocomposite film is fabricated as a device demonstration for practical application. This work manifests a new approach to breaking the intractable trade-off relation between the enhanced permittivity and decreased breakdown strength of the nanocomposite and achieving high-performance composite dielectrics for capacitive energy storage.</p>