Abstract: Several drill pipe washout accidents of G105 steel grade occurred successively in an oil well. The causes of washout of the drill pipes were analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination, mechanical property test, micro analysis, coating quality evaluation, energy spectrum analysis and so on. And the stress distribution in the intermediate belt of the drill pipe was simulated by finite element method. The results show that the inner coating of the drill pipes foamed and fell off under the long-term influence of the corrosion medium in the well, and the inner wall matrix of the drill pipes were corroded. Fatigue cracks produced in the corrosion pits of the intermediate belt of the drill pipes under alternating load in inclined section, and the crack propagation eventually led to washout of the drill pipes.