Abstract: Based on preceding researches，the displacing mechanism of CO₂ pad slug and the N₂ post slug injection about Gao ji low permeable reservoir in Jiangsu is studied for enhancing displacement recovery efficiency of gas displacing oil.This displacing process is different from conventional flue gas displacing oil.Enhancing displacement efficiency mechanism of CO₂／N₂combined slug injection is estimated and analyzed carefully after several times of important experiment，such as CO₂／N₂ intermiscible dilatational experiment to crude oil of Gao ji Oilfield，multiple-contact extraction experiment，MMP test in slime tube experiment of alternating CO₂／N₂slug injection，long core displacing experiment.Therefore，a profitable conclusion can be obtained that the displacing method of CO₂ pad slug and the N₂ post slug injection can improve dissolved expandability and displacing oil advantage of CO₂ under near miscible phase with N₂，displacing efficiency can be equal to or exceeds that of pure CO₂ injection.This method can not only benefit displacing efficiency of non-hydrocarbon injection，but also lessen producing strings and equipment corrosion problems when gas breakthrough of production well happens.On the other hand，it also can be used in displacing progress of injecting CO₂ pad slug before flue gas to increase oil recovery and realize CO₂ stored underground.

Key words: gas injection oil-displacing mechanism, displacing efficiency of gas injection, ways of gas displacing oil, CO₂／N₂ slug displacement, experimental methods of gas injection