CO2 flooding is an important part of the CO2 geological utilization process in CCUS. 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. Scanning electron microscopy, 3D confocal microscopy, EDS and XRD are 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.