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In this paper, an insulation flashover criterion model, in which the change of multi-conductor system coupling coefficient induced by impulsive corona is considered, is constructed by the electromagnetic transient analysis software ATP-EMTP. The model proposed, having taken the effect of the impulsive corona into account, is able to present the influence of conductor corona characteristics on potential difference of two ends of the insulator when an impulse flashover on insulator string occurs. A model of 220kV multi-circuit transmission line, which adopts insulation tower model with multi-wave impedance, is constructed as well. Thereby, it is feasible to study lightning back-flash proof levels (LBPL) and its influencing factors. The simulation result shows that the tower height plays a predominant role in deciding the performance of the counterattack lightning level of transmission lines. Moreover, the back-flash level of quadruple-circuit lines is remarkably lower than that of double and single circuit lines. It is suggested that, firstly the tower should be much lower if it is permitted; secondly the conductors at different heights in tower should adopt different insulation levels.