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In order to study the phenomenon of surface charge accumulation on cone-type insulators, a surface charge measuring system is established, based on the electrostatic probe method. The surface potential distributions on a cone-type insulator are measured and the charge distributions are calculated based on the 3D field calculation by the surface charge method. The characteristics of charge distributions are compared under different voltage durations and voltage amplitudes. The results show that the charge accumulation comes to a steady state after 120 min. A threshold voltage effect is observed that charges seldom accumulate unless the applied voltage reaches a certain magnitude. The mechanisms of charge accumulation are discussed. The surface conduction and volume conduction do not seem to dominate the charge accumulation on the insulator. Partial discharges in gas may be the main sources of surface charges.