Abstract: The fracture of 42CrMo steel functional joint produced by a company occurred during installation. The fracture reasons were analyzed by means of macroscopic observation and scanning electron microscope analysis, chemical composition analysis, metallographic examination and mechanical property test. The results show that the fracture property of the joint was brittle fracture, the segregation of the original structure of the material was serious, the quality and toughness of the material after heat treatment were not good, and the impact test results could not meet the design requirements. During the assembly process of the joint, the internal and external threads were not horizontally aligned, and they were subjected to abnormal loads during the installation process, and eventually broke.