Abstract: A 42CrMo steel triangular spiral drill pipe joint fractured during the coal seam drilling. The fracture causes of drill pipe joint were analyzed by means of macro observation, chemical composition analysis, mechanical property test, hardness test, metallographic examination and finite element simulation of stress distribution. The results show that the hardness of the 42CrMo steel triangular spiral drill pipe was too high because of low tempering temperature during heat treatment, resulting in low impact absorption energy and poor toughness. Under the action of alternating loads such as torsion and bending, the early fatigue fracture occurred at the stress concentration of the thread bottom.