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Investigations into surface flashover characteristics and underlying mechanisms in the range from atmospheric to near vacuum pressure are significant for developing high-power equipment based on the application of gases used for flashover prevention. In this work, polyetherimide (PEI) films with controllable outgassing properties were prepared. Then, the gas-related factors, namely, gas pressure, the type of gases, and outgassing were clarified by investigating the DC surface flashover of films in a wide pressure range (10−4–105 Pa) in different gas atmospheres. In all conditions, the variation of flashover voltage was found to be divided into three regions with pressure increasing, while its deeper reason is that the number density of molecules of the gas layer is affected by outgassing and ambient gas pressure, which affects the collision of electrons. In region I, the surface flashover voltage is closely related to outgassing and shows the highest value. In region II, the surface flashover is jointly affected by outgassing and ambient gas, along with a rapid decrease of flashover voltage with increasing pressure. In region III, the surface flashover is dominated by ambient gas, and the surface flashover voltage shows a U-shaped curve with increasing pressure. Transition points of the three regions shift significantly with the change of outgassing properties. In region I, the desorption of adsorbed gas has little effect on the surface flashover voltage, while the escape of dissolved gas dominates. Finally, a model of the diverse effects of gases on surface flashover in a wide pressure range was established. [ABSTRACT FROM AUTHOR]