Abstract: Through fragments collected from tests under explosive loading and observation of metallographic microstructure of the steel sections, the fracture mechanism of V-notches on the inner surface of 20 steel cylinder is studied. The result showed that the fracture mechanism of the case is mainly ductile fracture; quenched steel with high strength easily forms adiabatic shear band, which is in favor of controlling the shell into fragments of predetermined size and shape; the case fractures with the crack extending from the root of V-notch to the outer surface of the case.