Transmission line sag monitoring device

A transmission line sag monitoring device comprises a measuring host (1), an optical fiber composite overhead ground wire (OPGW) cable (2), a splice tray (3) and an optical fiber vibration sensor (4). The measuring host (1) is mounted in a transformer station communication machine room. The measuring host (1) is connected one optical fiber of the OPGW cable (2), and the optical fiber is led out of the splice tray (3) of a tower and is fixed on a phase line (5) with the optical fiber vibration sensor (4). The transmission line sag monitoring device is installed in the transformer station communication machine room, utilizes line OPGW optical fibers as signal transmission channels, and only needs one optical fiber vibration sensor for sag measurement while the sensor does not need on-site power supply. In addition, the transmission line sag monitoring device can be applied in the wild for a long time and has a simple measurement method.