Determining a reasonable amount of CO2 injection is critical to the reservoir engineering design and dynamic adjustment of CO2 flooding reservoirs. Considering the characteristics of CO2 flooding，the process of CO2 flooding is divided into three zones：CO2 flow zone，CO2-oil mixed zone and oil flow zone. Based on this model，a calculation model for the reasonable injection amount of CO2 flooding is established. Combining FICK law and laboratory experimental results，the key parameters of the model are calculated and the model is solved. The CO2 flooding charts of injection-production quality ratio with pressure level under different water production rate are established. It is found that，while maintaining a certain formation pressure，the underground volume of produced oil，CO2 and water is equal to the sum of the pore volume occupied by CO2 in the pure CO2 flow zone and the expansion volume of oil in the CO2-oil mixed zone. In the CO2-oil mixed zone，the diffusion rate of CO2 of miscible flooding is first fast and then slow. With the increase of pressure level，the injection-production quality ratio increases gradually，but the increasing trend became slower. The higher the water production rate，the lower the injection-production quality ratio. The reliability of this study is verified by combining with field test data. The reasonable injection amount of CO2 flooding reservoirs can be conveniently and accurately calculated with the research results，and which can guide the design of CO2 flooding scheme and dynamic tracking control.