Abstract: Aimed at the delayed fracture effect of 10B21 steel threaded shaft produced by the plant, the fracture reason was analyzed by means of optical emission spectrometer, optical microscope, Vickers hardness tester and scanning electron microscope. The results show that the surface hardness of the shaft was too high and the harmful hydrogen introduced by the process of surface carburizing and pickling was not eliminated in time, resulting in the hydrogen ductile-brittle fracture of the thread root under the synergistic effect of the screw pretention, thread root stress and local enrichment of hydrogen.