Abstract: The corrosion behavior of N80 pipeline steel in annulus protection fluid containing H₂S and CO₂ in an oilfield of South China Sea was studied by high temperature and high pressure dynamic evaluation system. The corrosion mechanism was analyzed by scanning electron microscope (SEM), energy dispersive spectrometer (EDS), Raman spectrometer and X-ray diffractometer (XRD). The results showed that in this environment, the corrosion products of N80 steel were mainly FeCO₃ and FeS, and the corrosion rate gradually decreased with the increase of corrosion time. The early corrosion process was controlled by H₂S and CO₂. The corrosion products were rough and not dense, and there were a large number of grooves and cracks. In the later stage of corrosion, the corrosion process were mainly controlled by CO₂. The corrosion product layer was gradually dense, and the particle size of the product increased, which could produce a certain barrier effect on the corrosive substances. In the process of CO₂ corrosion protection, the content of impurity gases such as H₂S should be strictly controlled to delay the development of corrosion.