The invention discloses a tightening device for an electric transmission line, and relates to the field of electric tools. The technical problem that insulator replacing work can cause insulator fracture easily in the prior art is solved. According to the tightening device, a connecting plate is arranged in the electric transmission line, connecting hardware fittings are arranged on the two transverse sides of the connecting plate, the connecting hardware fittings are arranged in a spaced mode in the longitudinal direction of the connecting plate, and the connecting plate is provided with an inner ring groove. The tightening device comprises a mounting body. A hook which is connected with the connecting plate is arranged at one end of the mounting body, the two longitudinal sides of the hook are close to the longitudinal side wall of the inner ring groove, and a connecting base which is placed between the connecting hardware fittings which are longitudinally spaced is arranged at the other end of the mounting body. The connecting base is connected with a tightening lead screw. An insulator can be replaced conveniently by tightening.