Abstract: When dry-type pulsed capacitors of large capacitance are used in vacuum, flashover in low pressure area is inevitable. This paper analyzes the mechanism and solution for the flashover of the capacitors in vacuum. The surface flashover voltage of three different insulating materials is theoretically and experimentally studied. Finally, an overall-encapsulated capacitor is put forward, which uses solid insulation to prevent discharge in low pressure area in vacuum. Experimental tests show that this type of capacitor has strong mechanical strength and excellent electrical performance, and thus can work reliably in vacuum (lower than 10-2 Pa).