Abstract: During the tripping process in an oil well, fracture occurred at the external thread of the drill pipe joint. The fracture reason of the joint was analyzed by means of macroscopic observation, chemical composition analysis, mechanical property test, metallographic examination, scanning electron microscope and energy spectrum analysis. The results show that the joint had the phenomenon of insufficient torque, staggering or deviation during the fastening process, which led to the abnormal stress distribution of the thread section. Under the action of complex alternating load in the underground, fatigue cracks were initiated at the bottom of multiple thread teeth in the stress concentration area, and the cracks continued to expand. At the same time, serious extrusion deformation occurred at the top of the tooth. The crack near the seventh tooth preferentially penetrated the inner wall of the joint to form a puncture channel. The drilling fluid invaded the thread meshing clearance through the puncture hole, resulting in the initiation of corrosion pits and secondary cracks on the surface of the tooth bottom, which accelerated the original fatigue crack growth rate. At the same time, the structural integrity of the seventh tooth area was destroyed, and the bearing capacity was significantly reduced. Finally, under the instantaneous load of tripping, the joint broke.