Abstract: In order to analyze the reasons for the severe external wall corrosion of a certain submarine multiphase pipeline, its 2.2 km pipe section with severe external wall corrosion was cut and deeply researched by means of macroscopic examination, chemical composition analysis, metallographic examination, hardness testing, energy spectrum analysis and X-ray diffraction. The results show that local corrosion in the form of nonuniformity appears in the external wall of the submarine pipeline, and the corrosion mode is mainly pitting corrosion, locally accompanied by deep corrosion pits. The reason for the local corrosion of the submarine pipeline is the failure of the anti-water cap. As the anti-water cap fails, seawater enters the thermal insulation and permeates through the damaged anticorrosion coating to contract the surface of steel pipe directly. Pitting corrosion is originated from the sulfide inclusions in the steel matrix, and induced and accelerated by Cl^－ in seawater. And deep corrosion pits are formed as some pitting corrosions expand to the inside and periphery of the steel matrix. The generation and development mechanism of local corrosion on the submarine pipeline is the autocatalysis effect and the oxygen concentration corrosion cell. The seawater inside the anti-water caps is concentrated to further accelerate the corrosion process, and the anti-water caps shield the cathodic protection current, so the enclosed corrosion environment is formed. As a result, it is difficult to control the local corrosion. The research results are of great significance for strengthening the operation management of submarine pipelines, ensuring the safe production of offshore oil and gas fields and improving the development efficiency.