Epoxy/Al₂O₃ composite is widely used as insulating material in power equipment. However, its performance in SF₆ gas under fast-rising pulse is little studied, which limits its application in guiding insulation design of pulse power devices. To investigate the insulation characteristics of epoxy/Al₂O₃ composite under nanosecond pulses, the flashover characteristics of this material are studied under pulse voltage with rise time of about 52 ns. The results show that its electric field of flashover is much higher than pure epoxy’s, and the flashover voltage is in accordance with Weibull distribution. The experiment shows that the flashover voltage increases significantly with the increase of the pulse steepness, from 108 kV at 5.8 kV/ns to 226 kV at 20.5 kV/ns. As pulse steepness increases, the flashover delay time decreases rapidly first, then tends to be flat. The surface morphology of the flashover area of the samples shows that the flashover channel is obviously carbonized and flashover discharge has a destructive effect on the composite material.