Investigations have been carried out on Alumina insulators in vacuum under an impulse voltage (0.7/4μS) in order to study the relationship between surface charging and surface preflashover phenomena of insulators in this paper. The Alumina insulators studied were 99.9% alumina cylindrical specimens and the employed electrode system is a non-uniform field electrode system. The surface preflashover phenomena were observed and the surface charge distribution of insulators was measured under varied impulse voltage levels and different shot number at a same fixed voltage level. Based on the experiments, it was found that the charge density and distribution is not only dependent upon the magnitude of the impulse voltage, but also the preflashover phenomena. The number of voltage shot has little influence on the charge density and distribution at the same voltage level if the surface preflashover phenomena can be observed during voltage shot.