Abstract: CO₂ corrosion is one of the important threats to the safe operation of oil and gas pipelines. Generally, the pitting often has a greater depth than that of the uniform corrosion and it is likely to be concealed, bringing about particularly serious hazards. In order to study the ion concentration distribution within the corrosion pits and verify the existing corrosion prediction models, the relative integral intensity was defined based on the characteristic peak of the bicarbonate in Raman spectroscopy and that of the background solution water with the internal standard method, and the quantitative relationship between the relative integrated intensity and the bicarbonate molar concentration was established in the range of 0.04-0.6 mol/L. Meanwhile, a set of flow cell system based on the in-situ laser Raman spectroscopy was designed, and the bicarbonate molar concentration was determined at different depths of artificial pits on the X65 pipeline steel in the flow environment. The experimental results show that: the bicarbonate molar concentration in the pit decreases gradually within 500 μm from the pit bottom to approach the bulk solution concentration as the distance increases, and it is basically constant when the distance is greater than 500 μm. The experimental method for determining the distribution of bicarbonate molar concentration in the pits could provide important reference for the development of pitting theory and the verification of corrosion calculation models.