Abstract: Q345R+S31603 extra-thick（substrates 90 mm+composite 3 mm） composite steel plate produced by a factory generates transverse cracking during the pre-bending process of the barrel. In order to find out the failure reasons, the fracture morphology, microstructure, chemical composition and mechanical properties of the cracked composite plate were comprehensively analyzed. The results show that the fracture mechanism of substrate is quasi-cleavage fracture and the cladding is dimple fracture. The heat treatment of carbon steel material is out of control, which results in the residual of widmanstatten structure in the metal structure, the material’s yield ratio is too high, and the impact toughness is not qualified.And there is a heat-cut pit in the side wall of the composite plate, which results in the stress accumulation. The crack starts from the defect and rapidly expands under the action of the huge tensile stress during the bending of the steel plate, which results in the fracture failure.