Abstract: This paper sets up the mathematical model of Taizhou Formation Reservoir of Caoshe Oilfield of Qintong Sag in the north of Jiangsu Province, and studies the proper injecting time for CO₂ flooding in this reservoir by using CMG software. The study shows that the earlier the reservoir performs CO₂ flooding, the more favorable for enhancing oil recovery; the latest injecting time for CO₂ flooding is when the water cut is not more than 70%, and Taizhou Formation is still in the favorable stage for gas injection. When the water cut is lower than 70%, the ultimate recovery factor of different injecting time is declining in the linear trend; but when the water cut is higher than 70%, the decline of ultimate recovery factor is large and the effectiveness of CO₂ flooding is obviously declined.