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During the developing process of flashover across an insulator in vacuum, the insulator surface is usually charged. It is of great importance to analyze the charging phenomena for better understanding the flashover characteristics in vacuum. It is considered that there are two kinds of mechanisms closely related to the surface charging phenomena of insulating materials, i.e., the classical secondary electron emission occurring over an insulator, and charge injection and accumulation occurring inside an insulator. Based on the rigorous analysis of the kinetic processes of both primary and secondary electrons, the related surface charges were analyzed. In this process, the influence of image force of mirror charge was taken into account. Involving the detrapping of charges trapped and the recombination of charges injected, the charging process due to charge injection and accumulation was also deduced theoretically. Some formulas were given to express the density of surface charges.