An impact device, characterized in that the impact device comprises: a transmission shaft which has an impacted section, an extending section extending from the impacted section along an axis of the transmission shaft, a first protrusion extending from the impacted section toward a first radial direction, and two first concave surfaces (34A, 34B) located at two sides of the first protrusion; an impact cylinder which is provided at an inside thereof with a passage for accommodating the impacted section of the transmission shaft, at an outside thereof with two elongated protrusions (43, 44) which are located at two opposite sides of the impact cylinder and extend parallel to the axis, and at two other opposite sides thereof with two first radial holes, the two first radial holes and the two elongated protrusions (43, 44) being alternately arranged along a circumference of the impact cylinder; two first impact pins (51 A, 51B) which are respectively received in the first radial holes, shaped as circular columns extending parallel to the axis of the transmission shaft, and abuttable against the first concave surfaces (34A, 34B) of the transmission shaft; and a first ring which is sleeved onto the impact cylinder and the first impact pins (51A, 51B), and provided at an inside thereof with a first narrow groove and a first wide groove both extending parallel to the axis of the transmission shaft, wherein the first wide groove is wider than the first narrow groove; one of the elongated protrusions (43, 44) is located in the first narrow groove and the other elongated protrusion (43, 44) is movably located in the first wide groove. a transmission shaft which has an impacted section, an extending section extending from the impacted section along an axis of the transmission shaft, a first protrusion extending from the impacted section toward a first radial direction, and two first concave surfaces (34A, 34B) located at two sides of the first protrusion; an impact cylinder which is provided at an inside thereof with a passage for accommodating the impacted section of the transmission shaft, at an outside thereof with two elongated protrusions (43, 44) which are located at two opposite sides of the impact cylinder and extend parallel to the axis, and at two other opposite sides thereof with two first radial holes, the two first radial holes and the two elongated protrusions (43, 44) being alternately arranged along a circumference of the impact cylinder; two first impact pins (51 A, 51B) which are respectively received in the first radial holes, shaped as circular columns extending parallel to the axis of the transmission shaft, and abuttable against the first concave surfaces (34A, 34B) of the transmission shaft; and a first ring which is sleeved onto the impact cylinder and the first impact pins (51A, 51B), and provided at an inside thereof with a first narrow groove and a first wide groove both extending parallel to the axis of the transmission shaft, wherein the first wide groove is wider than the first narrow groove; one of the elongated protrusions (43, 44) is located in the first narrow groove and the other elongated protrusion (43, 44) is movably located in the first wide groove.