Tuning the structure-activity of fillers and matrix is crucial for designing polymer-based dielectric capacitors with high energy storage performance. Up to date, how the fillers’ structural characteristics (surface/interface configurations, dimensions, orientations etc.) contribute to the overall energy storage is far from unveiled. To this end, a combined filler-polymer dual-side design strategy is developed, which involves the DFT guidance for the electronic transport criteria for the designable synthesis of KNb₃O₈ fillers. Four different structural configurations are constructed, which are surface-modified with polydopamine (PDA) to fabricate the final composite films, i.e. PDA@KNb₃O₈/PVDF-P(VDF-HFP)-PMMA with particular orientations and arrangements, through a well-controlled solution casting method. Comprehensive structural and electrical investigations reveal that 1D/2D-orientated PDA@KNb₃O₈ fillers could obviously enhance the breakdown field and energy storage performance. The difference is that the 1D fillers more effectively improve the energy efficiency (up to 72%), while the 2D fillers more steadily achieve high energy density (U_e = 28.35 J/cm³) among the highest U_e reported for the composites. This work not only uncovers the structural origin of the electrostatic storage in inorganic-polymer composite films but also provides critical insights in designing high-energy-density film capacitors.