Abstract: In order to reveal the developing progress of flashover in Dodecylbenzene and study the effective methods to improve the creepage distance of pulsed power devices. Flashover voltage of Plexiglass and nylon6 were measured under negative pulse voltage through experiments with different pulse risetime and different electric form. By the result of experiment, it is found that the flashover voltage increase as impulse steepness increases. With the flashover distance increasing, the flashover voltage increases, but flashover field strength decreases. The more nonuniform electric field is, the more prone to flashover occurs, and when 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. The experiment results can also provide methods for insulation design of pulsed power devices.