Abstract: Through the detection and analysis of the operating conditions, corrosion gas, cleaning product composition and microbial content of oil-gas-water mixed transportation submarine pipelines in an oilfield of South China, combined with corrosion simulation tests and effectiveness evaluation test of corrosion inhibitor, the causes of internal corrosion of the pipelines were determined. The results show that the cleaning products were mainly CO₂ and H₂S corrosion products, calcium salt scale and a small amount of sand. Under the condition of only containing CO₂, H₂S and corrosion inhibitor, the main corrosion form of the inner wall of submarine pipeline was overall corrosion. On this basis, microorganisms played a significant role in accelerating corrosion, resulting in serious localized corrosion under the deposits.