Abstract: With the development of offshore oil and gas fields, the oil–water two-phase mixing flow-transmission technology has been widely used in subsea pipelines. The high water cut and multiphase flow regime induce harsh and complex corrosion conditions; hence, mild steels combined with corrosion inhibitors are used in the construction of offshore pipelines for corrosion control. However, corrosion failure cases show that severe localized corrosion constantly occurs at the oil–water interface in oil–water mixed transmission pipelines, and the understanding of the mechanism and inhibition effect of corrosion inhibitors is limited. Moreover, laboratory studies on CO₂ corrosion problems in oil–water mixed transmission pipelines usually consider only pure-water systems to simulate the corrosive environment. These studies seldom regard the effect of the oil phase on the corrosion process even though the actual production and transportation of fluids often involves multiphase mixed media. The oil phase is one of the important factors that affect corrosion behavior. Studies on the impact of the oil phase on the inhibition effect of corrosion inhibitor are still relatively lacking. Further studies on the inhibition effect of corrosion inhibitor in oily corrosive environments of oil–water mixed transmission pipelines are needed. In this study, the inhibition effect and mechanism of a corrosion inhibitor at the oil–water two-phase interface under flow conditions were investigated using the rotating cylindrical electrode (RCE) technique combined with electrochemical methods (electrochemical impedance spectroscopy and polarization curve analysis), laser scanning confocal microscopy, scanning electron microscopy, and UV-VIS spectrophotometry. The results reveal that 100 mg·L⁻¹ of seventeen alkenyl amide ethyl imidazoline quaternary ammonium salt as a corrosion inhibitor in carbon steel for an aqueous phase of the oil–water two-phase stratified medium exhibits significant inhibition effect, and the corrosion inhibition efficiency reachs 99%. However, the effective mass fraction of the corrosion inhibitor decreases to 31% before mixing at the oil–water interface because of the presence of oil. As a result, the corrosion inhibition efficiency is only 83%, and the inhibition effect is poor; moreover, the corrosion of carbon steels cannot be effectively controlled. Further, significant groove corrosion is observed at the oil–water interface. Therefore, the corrosion of the sample in the oil area is slight, and the inhibitor can effectively inhibit the corrosion of X65 steel in the water area.