When the missile launcher was carried out during the third test procedure, fracture occurred to the 40CrMo bolt. In this paper, investigations were performed by macroscopic examination, electron microscope observation, metallographic examination and chemical composition analysis in order to get the fracture property and the reason of the fracture. The results suggest that the property of fracture is stress corrosion, as well as the original micro-cracks in the bolt before galvanizing and lead to stress concentration. During the test, the existence of bending stress and atmospheric oxygen, such as sulfur, chlorine, sodium and other interaction, which result in fracture of the bolt. Furthermore, high hardness values and a slight temper brittleness can also have a certain effect on the bolt fracture.