CO₂ flooding is an important part of the CO₂ geological utilization process in CCUS (carbon capture, utilization and storage). The CO₂ flooding and storage project carried out in the Yanchang Oilfield has achieved good results in increasing production and storing CO₂. But the reasons of pipe string corrosion failure are still unclear during CO₂ injection. SEM (scanning electron microscopy), 3D confocal microscopy, EDS(energy dispersive spectrometer) and XRD(X-ray diffraction spectroscopy) were used to characterize the corrosion morphology of the gas injection pipe string in the CO₂ flooding and storage demonstration area of Wuqi Oilfield. The pipe string corrosion products are analyzed to clarify the reasons for pipe string failure caused by CO₂ corrosion. The results show that local corrosion dominated by CO₂ corrosion causes corrosion failure of the pipe string. Corrosion products include FeCO₃, high-priced oxides of Fe and a small amount of FeS. Continuous injection of low-temperature CO₂ cannot cause corrosion. However, the increase in wellbore temperature and the return of formation water caused by stopping CO₂ injection can create conditions for CO₂/H₂S corrosion in the pipe string. In addition, pipe string corrosion is accelerated when gas injection wells are converted into water injection wells. It is recommended to strengthen the sterilization and anti-corrosion measures for the pipe string during the period of stopping CO₂ injection and switching to water injection.