In this study, the surface charge distribution on insulators in vacuum has been investigated under an impulse voltage (0.7/4μS) with a non-uniform field electrode system. The insulators studied were 99.9% alumina specimens with the shape of right circular cylinders about 5mm thick and 20mm in diameter. Using an electrostatic probe with shield in the form of capacitive voltage divider that was connected to an electrometer, the surface charge distribution on insulators were measured. The applied impulse voltage, and the corresponding current waveforms, the signal of luminosity were also observed during the voltage application. We investigated the surface charge distributions at different impulse voltage levels, at different shot number with a same voltage level, at the situations after a flashover occurred. Based on the experimental results, we found that the surface charge density on insulators is increased with the increase of the applied voltage, the shot number has a little influence at the same voltage level, and the charge density is decreased a lot after a flashover occurs. It's also found that the increase of charge density is often accompanied by the preflashover events.