The work aims to study the effect of different dissolved oxygen concentration on the pitting formation and development of in low alloy tubes in H2S/CO2 acidic environment. Corrosion rate was tested by performing corrosion weight loss experiment using high-temperature & pressure apparatus and normal pressure & temperature in simulated corrosion environment. Morphology of corrosion products and size of pitting pits were analyzed by using SEM. The dissolved oxygen had obvious effects on the corrosion products and produced great damage to the corrosion product film. The corrosion products covered on the substrate reduced, the product film thickness decreased and different sizes of pitting pits were found on the surface when the dissolved oxygen was present. The pitting phenomenon was obvious at the dissolved oxygen partial pressure of 0.1 MPa. The diameter of the pitting pits on the surface of L245NCS was nearly 0.4 mm and L290 0.25 mm. The addition of dissolved oxygen leads to pitting of carbon steel easily in the H2S/CO2 corrosion environment. Uniform corrosion rate increases significantly and corrosion products covered on the substrate decreased as the dissolved oxygen concentration increases. Due to the favorable oxidizability of dissolved oxygen, pitting corrosion potential may easily reach potential critical value and lead to the formation of pitting. Once pitting is formed, it will constitute a “small anode-large cathode” accelerated corrosion system along with the surrounding dense layer, thus producing deeper pitting pits.