In this paper, we studied the effect of filler conductivity on the surface insulation of composites. We proposed using different valence element doping as a strategy to prepare filler with gradient conductivity. The pristine BiFeO3 (BFO), Bi0.95Ca0.05FeO3, and Bi0.95Sm0.05FeO3 nanoparticles were prepared to fill into epoxy resin. The Rietveld refinement of the x-ray diffraction patterns was performed to investigate the transition of the lattice structure. According to the Kubelka–Munk function, the energy bandgap was calculated to verify the regulation of the BFO conductivity. Subsequently, we measured the flashover voltage of the composites with three types of BFO in ambient and vacuum atmosphere, respectively. We found the vacuum flashover is more sensitive to filler conductivity, and BFO with higher conductivity as filler is more conducive to surface insulation, whether in the air or vacuum. Finally, the optimal BFO bandgap of 2.1819 eV is determined for enhancing surface insulation of composites. Our research supplies a novel method for constructing high insulation composites for more wide applications and provides a scientific basis on flashover mechanism from the perspective of filler characteristics.