Abstract: Polymer-based dielectric capacitors are widely utilized in various fields, including defense, energy storage, new energy vehicles, power electronics, and medical applications, owing to their high breakdown strength, lightweight, flexibility, and ease of processing. However, the low dielectric constant of polymer-based film capacitors limits their energy density, thereby restricting their application in advanced technologies. To enhance energy density, this study adopts a casting method to incorporate ultra-low content titanium dioxide nanoparticles (TiO₂) into a polyetherimide (PEI) matrix, thereby preparing TiO₂/PEI composite films. Experimental results demonstrate that the incorporation of TiO₂ enhances the interfacial polarization of the composite films, leading to a significant increase in the dielectric constant (with a dielectric constant of 4.59@100 Hz when the TiO₂ content is 0.8vol.%, representing a 22.15% increase compared to pure PEI). Furthermore, TiO₂ doping suppresses charge migration and dendritic growth within the composite films, thereby improving the breakdown strength. The TiO₂/PEI composite film with 0.2vol.% TiO₂ exhibits a maximum energy density of 8.82 J/cm³ under an electric field of 400 MV/m, a 122.93% improvement compared to pure PEI. COMSOL simulation analysis reveals that the low TiO₂ content effectively reduces the leakage current density of the composite films, further enhancing the breakdown strength. This study provides an effective solution for the fabrication of high energy density polymer-based dielectric composite films, offering promising prospects for practical applications.