The invention discloses a composite die for punching equal-height stepped holes in the side face of a double-layer metal plate barrel. The composite die structurally comprises a box door, four supporting feet, an operating panel, a lower die seat, step drill bits and a composite die main machine. The composite die is characterized in that the box door is embedded into the left side of the outer surface of the lower end of the composite die main machine, the upper ends of the supporting feet are welded to the lower surface of the composite die main machine, the operating panel is embedded intothe right side of the outer surface of the lower end of the composite die main machine, the lower die seat is embedded into the middle of the composite die main machine, the step drill bits are embedded into the middle of the composite die main machine, the step drill bits rotate through a rotating mechanism, the step drill bits carry out punching through a punching mechanism, thus, the stepped hole working time of the step drill bits is shorter, the strength is more sufficient, and the stepped holes are prevented from deforming; and a pressing mechanism drives a cooling mechanism to move through gravity, the step drill bits are cooled after working, the temperature of the die is guaranteed, and the shape of the die is guaranteed.