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Charge accumulation has always been a problem for the safe operation of gas insulated switchgear (GIS)/gas insulated transmission line (GIL) both under AC and DC. It is of great significance to investigate the behaviour of surface charge under high voltage. In this study, the charge distribution characteristics and accumulation mechanism on insulator surface in air and heptafluorobutyronitrile/carbon dioxide (C(4)F(7)N/CO(2)) mixtures under AC voltage are studied via the means of surface potential measurement and inversion algorithm, combined with the improved method of controlling the truncated phase of AC voltage. The results show that under the needle–plate electrode structure, charge distribution on the insulator surface presents a three-tier concentric circle structure both in air and C(4)F(7)N/CO(2) gas mixtures, and the charge composition of the innermost circle is closely related to the truncated phase of AC voltage. Under the same amplitude of voltage, the range of charge distribution on insulator surface in C(4)F(7)N/CO(2) mixtures is smaller than that in air, with the negative charges domination. It is suggested that the characteristic of charge distribution in C(4)F(7)N/CO(2) mixtures is related to the larger electron attachment cross-section of C(4)F(7)N gas in a wide range of electron energy distribution.