Abstract: Through the examination of macroscopic morphology of cracks on a drill pipe and microscopic morphology of its fracture, EDS analysis, mechanical property test, chemical composition and microstructure analysis, the washout reasons of the drill pipe with φ127 mm×9.19 mm in size made of G105 steel were analyzed. The results show that the washout of the drill pipe was ascribed to early corrosion fatigue. There were some corrosion pits on the outer wall of the drill pipe body due to the corrosion of weak acid fluid in downhole, which caused stress concentration at the bottom of corrosion pits. Under the action of alternating stress, multiple cracks quickly initiated at the bottom of corrosion pits. The bit jumping during drilling process and serious cross-section mutation of combined drill stem structure accelerated the expansion of fatigue cracks in the drill pipe. When one of the cracks penetrated the pipe wall, washout happened in the drill pipe.