Maintaining high charge/discharge efficiency while enhancing discharged energy density is crucial for energy storage dielectric films applied in electrostatic capacitors. Here, a nano-submicron structural film comprising ferroelectric material P(VDF-HFP) and linear dielectric material PMMA has been flexibly designed via the electrospinning process. Nano-submicron structure enables the film to maximize the ferroelectric material component and obtain improved dielectric performance without sacrificing breakdown strength and charge/discharge efficiency. As a result, the 40%-420 nm PMMA-P(VDF-HFP)@PMMA sample achieved an discharged energy density of 13.72 J/cm³ at a field of 740 kV/mm, with an impressive charge/discharge efficiency of 80%. This work presents a composite dielectric film that excels in breakdown strength, discharged energy density, and charge/discharge efficiency, offering a strategy for designing reliable, industrial-grade energy storage dielectrics.