Abstract: In order to reveal the developing progress of flashover in dodecylbenzene and study the effective methods to improve the creepage discharge of pulsed power devices,fashover voltag of polymethyl methacrylate and nylon6 were measured under negative pulse voltage through experiments at different pulse risetime and different electric form. Form the results of the experiments, it is found that the flashover voltage increases as impulse steepness increases. With the flashover distance increasing, the flashover voltage increases, but the flashover field strength decreases. The more nonuniform the electric field is, the more the flashover occurs, and if there is a large distance, flashover voltage has a slowing trend with flashover distance increasing in a very nonuniform field, and there is a clear inflection point. It is believed that they have similar flashover mechanism in vacuum and insulation oil and the flashover is completed in a gasifying path.