Phase of CO ₂ flooding is studied, combining with MMP and maximum gas injection pressure in the study area. The results show that the MMP is 22.2 MPa. The fracture pressure of formation is 33.88 MPa. The maximum gas injection pressure of wellhead is 15.8 MPa. The gas injection pressure is 10.5 MPa, and corresponding injection gas pressure of well bottom is 19.1 MPa. CO ₂ flooding belongs to immiscible flooding. When the gas injection pressure of wellhead is increased to 15.0 MPa, the bottom hole pressure is 23.6 MPa, which is higher than the minimum miscible pressure, there is miscible zone near the well area in the gas injection well.