The invention relates to a clamp. The clamp comprises a base assembly and a clamping head. The base assembly comprises an end cover, a rotating shaft and a piston. The end cover is mounted on the rotating shaft to enclose a sliding cavity in a sealing manner; and a through hole is formed in the centre of the end cover. The piston is placed in the sliding cavity and divides the sliding cavity intoa first chamber and a second chamber. The outer peripheral surface of the piston is in dynamic sealing connection with the inner wall of the sliding chamber; and the first chamber is connected with anair source through an air channel. The centre of the piston is protruded outwards to form a boss which is adapted to the through hole. The clamping head comprises a mounting part and a cylindrical clamping part which is arranged on the mounting part and is used for clamping a machined workpiece. The mounting part is mounted on the end cover and is provided with a central hole. At least two through grooves are formed in the outer peripheral surface of the mounting part at an interval; and through the formation of the at least two through grooves, the overall cylindrical clamping part can expand or close. The boss extends out from the through hole to jack and press the mounting part outwards so as to expand the cylindrical clamping part; and the cylindrical clamping part is closed when theboss retracts from the through hole.