The invention discloses a cap tightener of a full-automatic capping machine. The cap tightener comprises a connecting sleeve which is in power connection on the capping machine. An anti-rotating rod and a lifting rod are rotatably connected to the upper portion and the lower portion in the connecting sleeve respectively. A fixing seat is connected to the lower portion of the outer wall of the connecting sleeve, a plurality of extrusion roll swing arms and hemming roll swing arms are hinged to the fixing seat, and a swing drive mechanism for respectively driving the extrusion roll swing arms and the hemming roll swing arms are further mounted on the connecting sleeve. A cap pressing block is mounted at a lower extending end of the lifting rod, and a cap feeler lever which can slide up and down penetrates in the cap pressing block. An extrusion roll is connected to the bottoms of the extrusion roll swing arms, and an extrusion roller is mounted at a lower extending end of the extrusion roll. A hemming roll is connected to the bottoms of the hemming roll swing arms, and a hemming roller is mounted at an extending end of the hemming roll. A sliding ring sleeved on the outer side of the cap pressing block is in power connection onto the cap feeler lever. The cap tightener is simple in structure and flexible to adjust, can effectively protect a bottle and has the advantages that the cap tightener is not sealed with a cap and capping efficiency is high.