Abstract: In the process of shooting test, a batch of S20A alloy structural steel cartridge shells fractured transversally. Methods such as chemical composition analysis, macro analysis, hardness test, metallographic examination, scanning electron microscope analysis and so on, were used to analyze the fracture reasons of the cartridge shells. The results show that the reason for the fracture of cartridge shells should be the recrystallization occurred in the local heating area of the shell during the process of shooting test, and the hardness of the region was reduced. At the same time, the shell fracture was caused by the high speed shear force.