The work aims to analyze and confirm the corrosion causes by component test and electrochemical test of external corrosion products in the surface casing of A2 well and prepare the anti-corrosion measures according to the analysis result and the actual situation of the site. Energy spectrum element analysis and XRD component analysis were carried out on the collected corrosion product, and SRB and TGB analysis were also performed with reference to NACE-TM0194—2014 and SY/T 0532—2012. From the energy spectrum analysis, corrosion products mainly consisted of O, Fe and C elements, as well as a small amount of Na, K, Cl and S elements. XRD analysis showed that the main components of corrosion products were BaSO4, Fe2O3 and FeS. Bacteriologic examination revealed that the corrosion products contained SRB and TGB, which provided the conditions for H2S through microbial metabolism. The external corrosion products of the surface casing of A2 well are mainly Fe2O3 and FeS. The main cause of corrosion is oxygen corrosion, and minor cause is secondary sulfide corrosion generated by bacterial action. A large amount of barite and a small amount of inorganic salts in the product are from drilling fluid, and seawater provides a source of bacterial. Based on the results of corrosion product analysis, the main anti-corrosion measure is to isolate the air and water from the surface casing. Based on the research and analysis of oil pipeline coating technology and combined with the survey of A2 well operation space, a kind of STOPAQ anti-corrosion plaster and coating protection scheme is provided. The scheme is safe and convenient to operate without special precondition on erosional surface and no decommission. The surface erosion has been obviously controlled by the actual testing of corrosion resistance measures for surface casing of the A2 well.