The accumulation and dissipation of surface charges play an important role in the insulation failure of the disc-type insulator in the gas insulated transmission line (GIL) and gas insulated switchgear(GIS). Introducing nanoparticles into epoxy resin to modulate dynamic behaviors of the surface charges and improve the flashover voltage is of great significance to ensure the safe operation of GIL and GIS. In this paper, epoxy resin/SiO₂ nanocomposites with mass fraction of 0%, 2%, 4%, 6% and 8% are prepared. With positive and negative DC voltages, the surface potential decaying process, trap distribution behaviors and flashover characteristics are obtained, a physical model of insulation failure is established based on modulating the trap distribution. The results show that the surface potential decaying rate is reduced with both positive and negative corona charging as the nano-SiO₂ is filled into epoxy resin, the minimum value is in the samples with the mass fraction of 4%. New hole and electron traps are introduced into epoxy resin by SiO₂ nanoparticles, which are with larger trap energy level and trap density, the maximum value is in the samples with the mass fraction of 4%. The flashover voltage of the specimen is enhanced by SiO₂ nanoparticles. Compared with pure epoxy resin, the flashover voltage of epoxy resin/SiO₂ nanocomposites with the mass fraction of 4% are respectively increased by 58.04% and 64.15% with positive and negative DC voltage.