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Surface charging commonly appears on dielectrics in vacuum in the presence of electron bombardment, seriously aggravating the superficial withstand strength of assorted devices. Nonetheless, a pre-conditioning technique is introduced in this paper capitalizing on surface charges to play an opposite role, enhancing flashover strength and suppressing the multipactor which is frequently found over vacuum insulator. A theoretical study is first performed, incorporated with particle-in-cell simulation to show the critical condition for a single-surface multipactor to initiate. Therewith, it is proven that a negative charge accretion in cathode adjacency can prevent the multipactor from commencing with efficiency. Subsequently, an analytical model is constructed to expatiate multipactor expansion with pre-set surface charges getting involved, illustrating an upper bound of its propagation velocity, influenced by pre-conditioning. Corresponding experiments are also conducted to corroborate previous conclusions, presenting both a deflected electron trajectory and an improved flashover threshold. In the end, a brief discussion is given on a possible method to generate desired surface charge distribution in practical applications. [ABSTRACT FROM AUTHOR]