It is difficult to carry out water flooding in volatile oil reservoirs due to the sufficient formation energy and the high initial pressure. CO2 flooding has been implemented in this type of reservoir for its excellent oil displacement characteristics. However，the higher dissolved gas oil ratio and the higher methane content contribute to a higher minimum miscible pressure（MMP）of CO2 flooding，which decreases the displacement performance. Dynamic miscibility characteristics and the injection timing of CO2 injection for volatile oil reservoirs are studied through laboratory experiment and numerical simulation. The results show that the characteristics of releasing methane and decreasing CO2 MMP is evident in the CO2 flooding after a depletion production of volatile oil reservoirs. That is to say，methane partially releases from the crude oil as the formation pressure decreases，which is favorable to the decreasing of CO2 MMP. The extent of decreasing CO2 MMP is closely related to the crude oil type and initial dissolved gas-oil ratio. When the crude oil is closer to the condensate oil，a higher dissolved gas-oil ratio results in a lager decreasing degree of MMP. On the contrary，when the crude oil is closer to black oil，a lower dissolved gas-oil ratio results in a smaller degreasing degree of MMP. The CO2 injection timing after the depletion of volatile oil reservoirs is proposed according to the dynamic miscibility mechanism. It is found that as the dissolved gas-oil ratio increases，the injection timing and the formation energy supplement decrease. Otherwise，the injection timing and the formation energy supplement increase.