Polymer nanocomposites have advantage over traditional materials in electrical properties from the standpoint of dielectrics and electrical insulation. The influences of nanoparticle dispersion in the matrix, which is mainly caused by different preparation methods, on the dielectric properties of composites have been given in the past work. In order to investigate the relationship between the dispersion of nanoparticles in the matrix and the dielectric properties of composites, nano-SiO2/epoxy composites are prepared by different methods. Nano-SiO2 is first modified by silane coupling agent to obtain nano-SiO2 powder and nano-SiO2 dispersing liquid, then unmodified and modified nano-SiO2 powder are mixed into epoxy by mechanical mixing method, and the modified nano-SiO2 dispersing liquid is mixed into epoxy by bubble mixing method to prepare nano-SiO2/epoxy composites. The amounts of nano-SiO2 content in the composites are 2 wt%, 3 wt%, 4 wt%, 5 wt% and 6 wt%, respectively. Breakdown strength and corona-resistance characteristics of the composites are tested. The results show that with the increase of the nano-SiO2 content, the breakdown strength and corona-resistance of nano-SiO2/epoxy composites increase. The maximal breakdown strength appears in the composites with 5 wt% nano-SiO2. This appearance accords with percolation theory. The composites prepared by bubble mixing method have better breakdown strengths and corona-resistances than the composites prepared by mechanical mixing method. The scanning electron microscope images of the nano-SiO2/epoxy composites are analyzed by Image J software to obtain the information about the nanoparticle number in the special grid. Morisita's index is used to characterize the dispersion of nano-SiO2 in the matrix quantitatively. It is concluded that the composites prepared by bubble mixing method have better dispersion than those prepared by mechanical mixing method. Compared with the unmodified nano-SiO2, modified one has good dispersion in the composite because of the improved compatibility between the nanoparticles and the matrix. Based on the role that nano-SiO2 particles block discharge from developing in the composite, the better dispersion means that there are more nanoparticles and more barriers on the discharge path. Meanwhile, the better dispersion also means that more interface areas form between nano-SiO2 and matrix. The shallower traps supplied by the interface area will contribute less energy when current carriers jump into and out off the traps. So the better the dispersion of nano-SiO2 in the matrix, the superior the breakdown strength and corona-resistance of the composites are.