<p>Polymer-based dielectrics are widely employed in electrostatic energy storage capacitors serving as pulse power supply owing to their lightweight nature and rapid charge-discharge capability. However, their intrinsically low dielectric constant severely limits energy storage density. Although high-dielectric-constant nanofillers are commonly incorporated to enhance permittivity, organic-inorganic interfacial incompatibility often induces particle agglomeration and structural defects. In this work, we propose a confined co-doping strategy for structured polymer dielectrics, wherein BaTiO₃ and Al₂O₃ nanoparticles are co-doed within the ferroelectric core P(VDF-HFP) of coaxial fibers and undergo self-assembly. This approach simultaneously enhances both energy density and charge-discharge efficiency. As a result, the 1 wt% BaTiO₃/1 wt% Al₂O₃ core co-doping composite dielectric achieves a discharged energy density of 19.2 J/cm³ and a charge-discharge efficiency of 81.0%, and maintains stable performance over 1 × 10⁵ cycles under an electric field of 400 kV/mm. This confined co-doping strategy thus provides an effective and scalable route for developing polymer-based dielectrics with high energy density and high reliability.</p>