Abstract: To enhance the surface insulation properties of polyimide films under direct current voltage conditions, this paper investigates the effects of plasma and chemical fluorination treatments, as well as their combined application, on the DC flashover characteristics of the films. Firstly, polyimide films were etched with plasma for different durations and chemically fluorinated under high-temperature conditions for varying lengths of time. Secondly, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were employed to observe the samples before and after treatment. Measurements and analyses were also conducted on their isothermal surface potential decay, surface conductivity, and DC flashover voltage. The results indicate that the two individual treatment methods have distinct mechanisms as follows: plasma treatment enhances the flashover voltage primarily by altering the surface micro-morphology, while fluorination treatment significantly improves the flashover voltage by improving trap distribution characteristics. Finally, the combination of plasma and fluorination treatments, in either order, demonstrates a higher flashover voltage enhancement compared to single treatments. In particular, the treatment sequence of fluorination followed by plasma not only physically improves insulation performance by forming bulk protrusions and a rough morphology but also chemically influences the trap distribution characteristics, reducing the density of deep traps. By combining the advantages of both methods, the flashover voltage is significantly increased.