The invention relates to a transmission line wires distributed arc sag monitoring device which comprises a computer, an information processing unit and an optical fiber composite overhead ground wire arranged on a transmission line, wherein one sensing optical fiber in the optical fiber composite overhead ground wire is a distributed optical fiber sensor. The transmission line wires distributed arc sag monitoring device further comprises a Brillouin optical time domain reflectometer and a spot type temperature sensor arranged on a transmission line wire. The distributed optical fiber sensor is connected with a first communication terminal of the Brillouin optical time domain reflectometer, a second communication terminal of the Brillouin optical time domain reflectometer is connected with the computer and a first data communication terminal of the information processing unit, the signal output terminal of the spot type temperature sensor is connected with the computer and a second data communication terminal of information processing unit. The transmission line wires distributed arc sag monitoring device is high in intelligent degree, automatic calculation is conducted through the computer conveniently, the distributed arc sag of the wire on the transmission line can be displayed in real time, and significant economic value and social value are achieved.