Abstract: A nail gun back cover includes a back cover shell body made of magnesium alloy. The inner side of the shell body includes a first cylinder and a second cylinder surrounding the first cylinder extensively formed therein. A groove is formed between the first cylinder and the second cylinder. In addition, a valve mechanism is formed in the shell body. The valve mechanism includes a fixed valve securely connected to the first cylinder and a movable valve contained in the groove. The movable valve can move relative to the fixed valve so as to control the air flow. Furthermore, some elastic bodies are used to touch the end surfaces of the groove and the movable valve. Moreover, a fixed ring is securely fastened to the back cover, so as to limit the range of movement of the movable valve. In this manner, the overall weight of the nail gun is reduced, making the nail gun much easier to operate.