Supercritical CO2can improve gas recovery and achieve carbon sequestration in tight gas reservoir. It is a hot topic but not applied extensively. The main restricts include the unclear mechanism of gas-to-gas displacement and the reduction of the natural gas asset value due to CO2breakthrough. In this paper, a novel experimental method is used, through constant-volume and constant-pressure experiments, to analyze the mixing process of CO2and natural gas in different parts of vessel. Moreover, the mechanism and impact factors of natural gas displaced by supercritical CO2are demonstrated in laboratory and numerical simulation. Based on the experiments, the diffusion coefficient of CO2in natural gas (CH4) is measured and input into the numerical model. The results show that supercritical CO2in the reservoir is ‘partially miscible’ with the natural gas. CO2displaces CH4to move upward due to the gravity differentiation, and CH4is produced in the process of CO2-subsidence. CO2flooding can enhance gas recovery by 18%, which verifies the technical feasibility of CO2flooding in tight gas reservoirs.