The invention relates to a power transmission line on-line monitoring system. The power transmission line on-line monitoring system includes a main control circuit, a temperature monitoring device, a video acquisition module, an icing monitoring device, a micro meteorological monitoring device, a wire vibration detector, a power source module, a radio frequency module and a wired communication module; the temperature monitoring device, the video acquisition module, the power source module, the icing monitoring device, the radio frequency module and the wired communication module are all connected with the main control circuit; and the micro meteorological monitoring device and the wire vibration detector are connected with the main control circuit through a wireless communication module. According to the power transmission line on-line monitoring system of the invention, a temperature sensor, a humidity sensor, a wind speed and wind direction sensor and the like are arranged on a whole power transmission line, and a power communication network is adopted, and therefore, the overall smart grid transmission line on-line monitoring system can be formed. With the power transmission line on-line monitoring system adopted, power grid transmission line accidents can be effectively prevented or reduced.