The life cycle of CCUS-EOR development can be divided into two parts. One is the simultaneous sequestration phase（SSP），and the other is the deep burial phase（DBP）of CCS. Determining the time length of SSP is an important part of CCUS-EOR development scheme design. According to the change trend of oil production under CO₂ flooding，SSP can be further divided into an oil production rising period，a stable oil production period，and an oil production declining period.The time length of the oil production rising period is calculated from the annual gas injection rate and the cumulative injection amount at the gas emergence time. The time length of the stable oil production period under gas flooding is calculated by means of“centralized recovery time of oil bank”. The change of stage recovery during the oil production declining period is studied by using the representative decline curves. The oil recovery rate during the stable oil production period and oil production decline rate under gas flooding are measured based on the concept of“oil production multiplier due to gas flooding”. Thus，a mathematical model for evaluating stage recovery in the case of CO₂ flooding is established. The moment when the stage recovery under CO₂ flooding approaches the estimated ultimate recovery is proposed as the transition point between SSP and DBP. On this basis，a phase transition criterion is introduced. The time length of SSP minus those of the oil production rising period and the stable oil production period is the time length of the oil production declining period.