Abstract: A radial insulation structure used in an intense beam diode was introduced and the surface flashover across the oil-solid interface was studied. The geometry structure parameters were optimized by simulating the static electric field intensity distribution on the surface of the cone-shaped insulator. The experiment was done on a pulse power source with 40 ns pulse width and 100 Hz repetitive frequency. A bush-like streamer was observed on the insulator surface after 100 000 shots loaded in 18 months. The streamer root region of the insulator was severely carbonized and the breakdown and carbonization happened in the depth direction of the insulator, leading to the formation of pores. Based on the solid-liquid interface flashover mechanics, possible causes of flashover were discussed.