Abstract: Surface flashover that occurs along the gas-solid surface of the basin-type insulators is one of the essential failures of GIS/GIL, which significantly threatens the safe operation of ultra-high voltage transmission systems. To enhance the DC surface flashover characteristics of basin-type insulators, epoxy composites are treated by ultraviolet irradiation in a vacuum, and various techniques, such as surface potential measurement and quantum chemical simulation, are adopted to obtain the distance of molecular chain, density, thermal stability, energy level band, trap parameters, surface charge distribution, and DC surface flashover voltage; moreover, the influence of ultraviolet irradiation on DC surface flashover is clarified. The results show that the DC surface flashover voltage of the 12-hour ultraviolet irradiated epoxy composites shows a 17% increment compared to the untreated sample. After surface hetero-charging for 6 h with a +20 kV voltage, the DC flashover voltage increases by 24%. The DC surface flashover is decreased by less than 2% six months after treatment, which exhibits long-term stability. The analysis of trap level and charge transport characteristics indicates that high-energy photons accelerate molecular chain crosslink after ultraviolet irradiation in a vacuum. The crosslinking structure facilitates the LUMO level of epoxy composites to decrease, thus large amounts of deep traps are introduced, which improves the energy level and density of epoxy composites. The increase of deep trap level causes charge to accumulate near the electrode, which impedes carrier injection from electrodes, therefore, the electric field above the solid surface is reduced, and the DC surface flashover voltage is increased. Based on the relationship among ultraviolet irradiation in a vacuum, charge transport, and surface flashover, ultraviolet irradiation in a vacuum can be an essential strategy in improving DC surface insulation of GIS in the future.