The utility model discloses a tapered screw-thread fit device for fixing a core rod of a composite insulator of an overhead power-transmission line, and the device comprises a connecting sleeve. The upper end of the connecting sleeve is provided with a tapered thread hole. The interior of the tapered thread hole is provided with a finger-shaped tapered external threaded pipe in screw-thread cooperation, and is also provided with a clamping hole which cooperates with the core rod of the composite insulator in a diameter cooperation manner. The device provided by the utility model is simple in structure, and is easy to install. A core rod clamping strip can firmly press the core rod of the insulator, thereby effectively preventing the severe accident that a wire falls to the ground, and adverse effects, wherein the accident and the adverse effects are all caused by the brittle break of the core rod of the composite insulator.