Abstract: Charge accumulation on the insulator surface is one of the main factors that lead to insulator flashover, which affects the stable operation of power systems. In order to study the characteristics of surface charge accumulation on spacer in gas-insulated transmission line (GIL) under AC voltage, we set up a surface potential measurement platform of the simplified post insulator based on the active electrostatic probe method. By the means of COMSOL simulation and static electric field calculation, the transfer function matrix of the truncated cone-type insulator was achieved, and the charge distribution was obtained by inverse calculation. The accuracy of the inversion algorithm was verified by comparing the preset charges and the result of calculation. At the same time, the relationship between voltage phase and charge distribution on the insulator surface was obtained by extracting the truncated phase of AC voltage. The results show that positive and negative charges co-exist on the insulator surface, and there is a denser 'charge band' near the high-voltage electrode. When a wire is arranged on the insulator surface, a hetero-polar 'charge pairs' appear at the ends of the wire. Both the polarity of 'charge band' and 'charge pairs' are related to the truncated phase of AC voltage, and the charge density is affected by the voltage amplitude and the truncated phase.