Gas-insulated equipment represents a vital component in HVDC transmission projects. Currently, epoxy resin post insulators are prevalently employed for supporting insulation within such equipment like DC wall bushing and DC gas-insulated transmission line. Nevertheless, flashover failures induced by accumulation of surface charge pose a significant risk to long time operation of these epoxy post under DC electric fields. Consequently, the problem of surface charge on epoxy post surface at HVDC gas-insulated systems and its influence on flashover has garnered extensive attention. To solve the difficulty, the flashover voltage of post insulator under rapid rise DC voltage with −2.6 kV/mm prestressed, similar with the real average electric field in DC equipment, for 25 h to 100 h were tested in this work. The surface charge accumulation mechanism was adopted to analyze the flashover process. The results indicate when the prestressed duration increase, the accumulated negative charge will decrease the total electric field nearby the HV electrode, and decrease the initial electron to trigger impact ionization. In conclusion, there could be a 5% growth in DC flashover voltage when prestressed. The findings of this paper can provide a reference for analyzing the effect of operation duration on flashover within inner-insulated devices.