Objective To investigate the external corrosion for two excavation points of an oil pipeline in southwest China. Methods Through the on-site testing and laboratory analysis on phys-chemical properties of soil and composition of corrosion product. Results The soil is slightly basic for the excavation point A, and its corrosivity is weak, as a result, where the pipeline suffer weakly uniform corrosion, and the corrosion products are mainly iron oxides, including FeO( OH) , Fe( OH)₃ and Fe₃ O₄ , also the cathodic reaction is reduction of dissolved oxygen. While for excavation point B, because of the strong acidity and high content of sulfate ion, the soil is highly corrosive and makes the pipeline suffer serious pitting corrosion, and corrosion products are mainly composed by FeSO₄ ·7H₂ O, also contains a small amount of basic ferric sulfate, moreover the cathodic process is mainly the reduction of hydrogen ions. Conclusion Due to the significant difference of the phys-chemical properties of soil, there are notably different on the corrosion morphology, products and mechanism of pipeline between two excavation points.