Abstract: Analyzed in the essay are the causes for the piercing failure of the S135 drill pipe by means of methods like geometric dimension measurement，piercing fracture morphology analysis and mechanical property testing，and consideration of the loading state and the service conditions of the drill pipe.Accordingly，five preventative actions are proposed.The analysis result leads to the following conclusion.The piercing of the drill pipe is regarded as an early-stage corrosion fatigue failure.And the reason for such failure is that under corrosion，and such loads as tension and compression，bending and torsion，fatigue cracks first occurred at the bottom of the corrosion pit on the external surface of the rather thinner part of the drill pipe，and then expanded to eventually get the drill pipe pierced.