The supercritical CO₂ corrosion problems of oil country tubular goods (OCTG) become increasingly prominent along with the application of CO₂ flooding enhanced oil recovery (EOR) technique and the exploitation of deep oil wells under high temperature and high pressure. Actually, OCTG steel often suffers from multiphase fluid corrosion of crude oil, water and supercritical CO₂ at different stages of oil and gas production. However, studies about CO₂ corrosion of carbon steel used for oil and gas production generally are carried out considering only the aqueous phase without proper consideration of the oil phase that may be present. The crude oil in crude oil/water production environments is a key factor affected the corrosion behavior of carbon steel. Corrosion rate and corrosion type of J55 steel were investigated under the conditions of different oil/water ratios saturated with supercritical CO₂. SEM, EDS and XRD were employed to analyze the morphology and characteristic of corrosion scale on the steel. The corrosion models were developed to understand the corrosion mechanism with consideration of a variation of oil/water ratio in reality. The results show that uniform corrosion occurs along with the lower corrosion rate due to the protection of crude oil when the water cut of crude oil is within the range, i.e., 0~30% and the water-in-oil fluid is formed. But the local corrosion rate of the steel increases rapidly due to the inhomogeneous adsorption of crude oil with the fluid changing from water-in-oil to oil-in-water emulsion when the water cut is between 30% and 75%. The corrosion products deposited on the steel surface change the wettability of oil and water phase, therefore, water phase can preferentially wet the localized deposited scale, leading to the development of pitting corrosion under the scale. However, when the water cut is higher than 75% and the oil-in-water fluid is formed, water phase infiltrates the metal surface that blocks the corrosion inhibition of crude oil for the steel, hence, the corrosion rate increases dramatically. The localized failure of corrosion scale due to scouring action of the fluid and the dissolution of aggressive medium leads to mesa corrosion on the steel. When the water cut is 100%, serious uniform corrosion occurs as a result of the strong corrosiveness of supercritical CO₂ dissolved in the water phase. Furthermore, crude oil can weaken the dissolution of corrosion scale in the supercritical corrosive medium, which modifies the grain size, morphology and chemical composition of corrosion scale and improves the protection performance of the scale.