In order to study the corrosion mechanism of J55 carbon steel in supercritical CO2/crude oil/brine environment. An autoclave was used to simulate supercritical CO2/crude oil/brine corrosion environment with CO2 pressure of 9 MPa and temperature of 65 ℃. The average corrosion rate of J55 carbon steel with different water cut was measured. The micro morphology and element composition of corrosion product layer were observed and analyzed by SEM and EDS, and the maximum corrosion depth and pitting coefficient were measured. The corrosion model of J55 carbon steel in supercritical CO2/crude oil/brine environment was established and its corrosion mechanism was described. The results show that the average corrosion rate of J55 carbon steel in supercritical CO2/crude oil/brine system increases with the increase of water cut. When water content is less than 50%, the corrosion products are less, the maximum corrosion depth and pitting coefficient are small, and the corrosion morphology is uniform corrosion; with the increase of water content, the corrosion products increase, the maximum corrosion depth and pitting coefficient increase rapidly, and the corrosion morphology becomes local corrosion. The corrosion rate of J55 carbon steel in supercritical CO2/crude oil/brine system is much lower than that in supercritical CO2/brine system, and the inhibition effect of crude oil is obvious. The existence of crude oil changes the corrosion morphology.