A S135 drill pipe experienced severe corrosion failure only after several cycles in a well. In this paper, a series of experiments were conducted to analyze the causes of corrosion failure of drill pipe, including nondestructive testing, chemical and physical properties, macroscopic and microscopic analysis, and X-ray diffraction (XRD) analysis. The results showed that the corrosion failure of the external thread joint and pipe body of the S135 drill pipe belonged to oxygen corrosion, and a large amount of Cl⁻ in the environmental medium accelerates the corrosion. The external threaded joint of the drill pipe was more severely corroded than pipe body due to the following two reasons. Firstly, vortexing flow was easy to form at the drill pipe joints with single shoulder threads. Secondly, the drill pipe external thread joint had 3.0-grade banded structure segregation, forming a local small electrode potential difference.