The surface charge distributions under impulse voltage are measured. Two types of insulators are investigated. The influence of impulse voltage magnitude on the surface charge accumulation is studied. It is shown that the pre-discharge current has a neutralization effect on charge accumulation, which makes the surface charging very weak when the lightning voltage is close to the flashover voltage. The initial conditions of surface charge accumulation are discussed in the present paper, and the source of charging is also studied. It is concluded that partial discharge in gas near the surface of the insulator is the initial condition of charge accumulation under impulse voltage.