In the gas insulated switchgear (GIS), the existence of insulation defects will cause partial discharge, which results in degradation of the equipment and endangers the stability of the power system. The defect of surface discharge along the insulator is relatively severe among all kinds of insulation defects. In order to study the discharge development process and regularity of this kind of defect under constant power frequency voltage, this paper sets up a "triple-combination" model to simulate the surface discharge in SF₆. With the power frequency voltage kept unchanged at 95% of the flashover voltage, a set of partial discharge signal is recorded every 5 minutes until the flashover occurs frequently, and the statistical parameters of each stage is calculated. The results show that before flashover, the discharge development process of the surface model follows the sequence of the increase of discharge repetition rate, the increase of the discharge quantity, the intermittence of discharge and the rising of discharge again, and at last the flashover happens. The occurrence of discharge intermittence may have relationship with the strong electronegativity of SF₆, the uniformity of the electric field and so on. Therefore, long-term discharge detection is required to accurately determine the stage of equipment degradation, and measures should be taken accordingly.