The high-speed photography and the non-linear dynamics engineering program LS-DYNA are conducted to research on Al and Cu armatures expansion process under the drive of pressed composite B explosive with the charge method of several-section. A special experiment system with an equipment of blocking off armature fracture flashing light is built, and a thorough analysis of the “circular fracture” phenomena has been made: For the air gap existence, there is a difference between the axial movement velocity of the armature joint position wall and other positions wall. The upward wall forces the joint position wall and then the “circular fracture” appears. Some conclusions about Magnetic Flux Compression Generator design are obtained.