In high-voltage direct current (HVDC) transmission systems, electric charge accumulates on insulator surfaces, causing surface electric field distortion and flashover voltage reduction. Therefore, studying a material that can improve the insulator surface insulation strength is of great engineering value. In this work, several types of metal nanoparticles with different particle sizes and concentrations are doped into epoxy resin. The experimental phenomena enables some interesting conclusions: when no agglomeration of doped nanoparticles occurs, a higher doping concentration provides a better insulation performance. The larger the doping particle size is, the lower the insulation performance. Additionally, under the same conditions, different types of metal nanoparticles lead to slightly different results after doping. Especially after doping with low concentration (approximately 120 parts per million (ppm)) and small particle size (approximately 10 nm) nanocopper particles, the insulator surface charge accumulation was effectively suppressed, and the flashover voltage was significantly improved. Our analysis suggests that it may be related to the single-electron tunneling phenomenon. Relevant results provide a new way to improve the surface insulation strength of insulators in the future.