More than 60% corrosion failure of oil/gas pipeline happened in carbon steel pipeline transporting fluid containing CO₂. Adding appropriate amount of Cr to carbon steel can greatly suppress localized corrosion. So, anti-CO₂ low Cr steel and its corrosion mechanism become research hotspot. This work found that active-passive transition region occurred when low Cr steel was anodic polarized. But stable broad passive region like typical passivation was not found. This phenomenon was called semi-passivation. The anodic polarization behavior of low Cr steels with Cr content of 1%~5% and the effect of Cr content on semi-passivation in CO₂ corrosion environment had been studied and discussed respectively. The anode potentiodynamic polarization curves of five kinds of low Cr steels in solutions with different pH values had been tested, subsequently the critical pH value which made the occurrence of the semi-passivation of low Cr steels with different Cr contents had been explored. The components of the corrosion product films on the low Cr steel samples polarized in solutions with various pH values had been compared through the Raman spectroscopy. The results showed that with the increase of Cr content, the semi-passivation characteristics was more obvious. The pH value of solutions in the CO₂ environment contributes to the precipitation of Cr(OH)₃. The Cr³⁺ ions dissolved from the substrate generate the corrosion films of Cr(OH)₃ on the surface, resulting in the semi-passivation characteristics in the tests of anode potentiodynamic polarization.