Abstract Electrostatic dielectric capacitors are critical components in advanced electronic and electrical systems owing to their high-power density and ultrafast charge-discharge capability. However, achieving ultrahigh energy storage performance combined with robust radiation resistance remains a major challenge, particularly for practical applications in extreme environments. Guided by simulations, self-assembled nanocomposite films with dendritic-like structured ferroelectric embedded in an insulator are designed to overcome these challenges. This strategy boots energy storage performance by forming nano-polar regions and obstructing electric breakdown processes. More importantly, it not only exploits the intrinsic radiation-resistant properties of ferroelectric materials, but also takes advantages of abundant interfaces within the dendritic structure to enable a self-healing effect to improve radiation resistance. This self-healing mechanism, driven by interactions between ferroelectric and insulating phases, effectively eliminates radiation-induced defects and minimizes performance degradation under high radiation doses. Using this approach, we demonstrate the dendritic-like PbZr0.53Ti0.47O3-MgO nanocomposite film capacitor exhibits an ultrahigh energy density over 200 joules per cubic centimeter and an excellent radiation tolerance exceeding 20 Mrad. This work offers a promising approach for the development of advanced electrostatic capacitors, particularly for applications in radiation-exposed power systems.