In response to the increasing demand for miniaturization and lightweight equipment, as well as the challenges of application in harsh environments, there is an urgent need to explore the new generation of high-temperature-resistant film capacitors with excellent energy storage properties. In this study, we report an all-organic composite system based on two polymers with similar densities and high glass transition temperatures, achieving a synergistic effect of dielectric constant and breakdown strength. The preparation of the composite is simple, overcoming the challenge of dispersing nanoparticles in traditional organic-inorganic systems. The high polarity of polyethersulfone can modulate the polarization properties of the composites and, through a physical electrostatic effect, inhibit dipole relaxation, further reducing the current density of the composite dielectric at high temperatures, resulting in a significant improvement in insulating properties. The 9 : 1 composite dielectric at 150 °C demonstrates an energy storage density of up to 6.4 J cm-3 and an efficiency of 82.7%. This study offers a promising candidate material and development direction for the next-generation energy storage capacitors with broad application prospects.