Abstract: A radial insulation structure which is used in long pulse high current diode is introduced in this paper. The theory of vacuum flashover and the idea of design are briefly introduced. In the research, coneshaped insulator was used. The geometry structure parameters were optimized by simulating the static electrical field distribution. Experiment was done on a pulse power source with 200ns pulse width. The maximum voltage 750kV was obtained, and the average standoff electrical field of insulator is about 50kV/cm.