With the high-speed development of power grid

insulation failures of gas insulated switchgear（GIS） at higher voltage level are increasingly frequent during its service. As the feeble part of GIS insulation

the breakdown accident of pillar insulator caused by its surface flashover happens frequently. In order to explore the reasons for insulator flashover fault

with the help of 1 MV steep-fronted impulse generator

we studied the influences of gas pressure and wave front time on the flashover voltage of pillar insulator with metallic conducting particles attached on its surface under double exponential impulse voltage. Results show that when the gas pressure is increased

the surface flashover voltage of the pillar insulator appears as a hump shaped curve and the critical pressure corresponding to the maximum value of negative hump curve is lower than that of positive hump shaped curve. In addition

the flashover with positive voltage has a single channel while that of negative voltage is circuitous and has numerous branches. With increasing pressure

flashover channels of both polar gradually become connected. When the wave front time is changed

the 50%%flashover voltage of the pillar insulator appears as a "U" sharped curve. The critical wave front time of the "U" sharped curve under negative impulse is shorter than that under positive impulse. From the analysis

the hump shaped curve is caused by "corona stabilization"effect of space charges in streamers. Because of the differences in the development process of positive and negative streamers

the flashover channels behave differently too. The "U" sharped curve is caused by the combined effect of discharge time lag

space charges and charges injected by displacement current.