The influence of assembly cushion on the fracture of expanding metal cylindrical shell is studied. The velocity of the outer surface of cylindrical shell with or without cushion was obtained by DPS array, and the high-speed photography recorded images with obvious influence of cushion on the fracture of expanding cylindrical shell. The results show that the outer surface of the cylindrical shell in the cushion area has experienced the process of first convex and then concave compared with the area without cushion, which makes the radial displacement of the cushion covered cylindrical shell repeatedly misplaced, leading a final displacement of 0.34 mm lower than that in the area without cushion. This displacement difference may lead to radial shear fracture of the cylindrical shell. However, in the experiment, a crack appeared on both sides of the cushion/gap junction (7.5 degree along the cushion direction and 9 degree along the gap direction), which is produced by the disturbance caused by the transmission of two sparse stress waves generated by the cushion/gap boundary to the outer surface of the cylindrical shell. The fracture mode is different from both circumferential tensile fracture and shear fracture along 45 degree direction. This new fracture mode is closely related to the dynamic mechanical properties of cylindrical shell materials. Further numerical simulation analysis shows that the influence of the assembly cushion on the fracture mechanism of the cylindrical shell includes not only the addition mass effect, but also the change of the impact loading amplitude of the explosive acting on the cylindrical shell after passing through the cushion and the asynchronous difference between the impact loading sequence and other parts, and the influence of surface wave propagation caused by the interface between cushion and gap on the subsequent development behavior of cylindrical fracture mode .