Impact of compression and reinjection of the produced gas on displacement efficiency was studied and its mechanism was analyzed to solve the problem of how to recycle and make use of the large amount of CO2-rich associated gases during CO2 flooding. Based on CO2 flooding pilot test in Gao89 Block of Zhenglizhuang oilfield of Shengli oil province，the minimum miscibility pressure（MMP）between formation oil and CO2 was measured by long slim tube test method. The displacement efficiency of CO2 miscible flooding was determined by long core displacement experiment using pure CO2 and produced gases at various stages. The results show that when the CO2 content is 72.59%，the displacement efficiency is up to 76.9%，similar to that of pure CO2 miscible flooding. Moreover，the components of produced gas at different stages were analyzed，and the influence mechanism of gas composition on the displacement efficiency was investigated. The results suggest that the produced gas during CO2 flooding can be directly reinjected without purification. It provides a new and feasible solution to treat produced gas in the middle and late period of CO2 flooding.