Based on a three-dimensional self-consistent numerical model with consideration of electron scattering, trapping and transport, the charging effects due to low-energy defocused electron beam irradiation are simulated for a SiO2 thin film with a grounded conductive substrate. The results show that because of electron drift and diffusion, electrons can transit the electron scattering region, forming negative space charges. The space charge is, therefore, positive and negative within and outside the scattering region, respectively. Some electrons can flow to the conductive substrate, forming the leakage current, and the transient negative charging process tends to equilibrium as the leakage current increases. In comparison, the transient positive charging process approaches equilibrium with the number of returned electrons increasing due to the positive surface potential. In the equilibrium state, the surface potential of the film negatively charged decreases with film thickness and trap density increasing; it increases with electron mobility and dielectric constant. However, the equilibrium surface potential of the film positively charged varies slightly with film parameter.