Abstract: The bolt of an elevator traction machine broke in the process of use. The causes of bolt fracture were analyzed by means of chemical composition analysis, metallographic examination, hardness test and fracture analysis. The results show that due to raw materials, design, heat treatment and other reasons, the properties of bolt were reduced, stress concentration occured at the tooth root, and fatigue fracture occurred under the action of long-term alternating stress. The improvement measures of controlling the quality of bolt raw materials and strictly carrying out quenching and tempering heat treatment were put forward.