This invention provides an auto hammer comprising a housing, a head assembly including a striking device and transmission mechanism. The striking device includes a striking rod having a striking end. The transmission mechanism includes an impact wheel. The distance from the end surface of the striking end of the striking rod to the center of the impact wheel between 40 mm and 100 mm. This distance is arranged very small, in favor of the hammer being used in some narrow spaces. The auto hammer according to this invention meets the requirements of being used in narrow spaces by hands, thereby suitable for use in various circumstances.