A new automatic fastening device is introduced, aiming at the characteristics, techniques and disadvantages of the small-caliber cannonball fasteners. Through an indirect control of the depth and position of the fastening slot, both of the bullet pull strength and its steadiness of the small-caliber cannonball could be defined. An airdraulic reinforcing cylinder is need to drive the fastener, which could maintain the fastening pact and diminish the device volume. Application proves that the new automatic fastening device, whose feeding and blanking is convenient, is advanced design with a steady bullet pull strength and an admitted tolerance. It could also improve the quality of ammunition, ensure its power and could be well popularized.