A pneumatic hammer has a handle, a body, a valve seat, a cylinder device, an assembling device, and a chisel. The body is transversally connected to the handle and has a chamber. The valve seat is mounted in the chamber of the body and has a rear valve, a front valve, and a valve plate disposed between the rear valve and the front valve. The cylinder device is mounted in the chamber, abuts against the valve seat, and has a cylinder barrel and a hammer. The cylinder barrel is mounted in the chamber and has a cylinder chamber. The hammer is movably disposed in the cylinder chamber. The assembling device is connected to the cylinder device, and has a mounting jacket, an engaging element, multiple positioning elements, a spring, and an engaging ring. The chisel is detachably connected to the assembling device, and is inserted into the cylinder chamber.