Injecting flue gas directly into depleted reservoirs cannot only recover the remaining oil in the reservoir and reduce the cost of flue gas treatment, but also bury the harmful gas and achieve the purpose of energy saving and emission reduction. Minimum miscible pressure (MMP) is a key parameter to determine whether the reservoir achieves miscible displacement. In order to achieve miscible displacement, it is of great significance to study the influence law of flue gas components on MMP and optimize the injection gas components. The accuracy of the improved MMC model is verified with Cases; the order of influence of impurity gases in CO2 on MMP is investigated; and the improved MMC model is used to optimize the injected gas components under the mixing mode of CO2 and flue gas with MMP as the target. The results show that: the improved MMC method has higher accuracy and faster calculation speed; the primary order of the influence of impurity gases in CO2 on MMP is N2 > CO > O2 > H2S > SO2; for Case 1, the miscible displacement of the gas injection project can be realized by controlling the CO2 content of the injection gas to be more than 65% under the pressure of the reservoir (e.g., 40 MPa). This study can provide a theoretical basis for the extraction of crude oil from depleted reservoirs and the geological storage of flue gas.