Abstract: During the operation of a well, the drill pipe was stuck, and the fracture accident of S135 steel drill pipe occurred during the unlocking process. The causes of drill pipe fracture were analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination, mechanical property test, scanning electron microscope and energy spectrum analysis. The results show that during the service process, the friction heat was generated by the friction between the drill pipe and the borehole wall, and the temperature exceeded the phase transition temperature of the material, which caused the material to undergo austenite transformation and form a banded structure of proeutectoid ferrite and martensite, resulting in a decrease in the mechanical properties of the material. The cooling rate of the inner and outer walls of the drill pipe was different. Under the combined action of structural stress and thermal stress, the hydrogen atoms in the material accumulated and diffused in the martensite strip near the inner wall, resulting in stepped cracks in the material, which eventually led to the fracture of the drill pipe.