Abstract: In order to study the fracture behaviour of the welded joint of explosive B30\alloy clad QT-steel plate,many types of COD specimens with various crack-direction were prepared and tested.From the test results,it is concluded that for the specimens of which the crack-plane is parallel or inclined to joint interface and the crack-tip is at or in the vicinity of the interface,the toughness values will be decreased or increased in different amounts as compared to those of the metal layer which thecrack-tip is located in.Thus a hypothesis model named "Crack-tip plastic-zonc abnormal development model" was proposed and for those specimens of which the crackplane is perpendicular to the interface,the main cause of "crack-arresting action" is considered to be the shearing of the metal at the clad interface,besides the formation of plane-stress state.This phenomenon can be observed by SEM fractography.