The embodiment of the invention provides an online monitoring and early warning system for the galloping of a power transmission line, and the system comprises a galloping monitoring device, a micro-meteorological monitoring device, a camera device, a monitoring base station, and monitoring center equipment. The monitoring base station transmits the meteorological information collected by the micro-meteorological monitoring device, the motion data collected by the galloping monitoring device and the image information collected by the camera device to the monitoring center equipment. The monitoring center equipment judges whether an environment can cause the galloping of the power transmission line or not according to the meteorological information, and calculates the motion track of the power transmission line according to the motion data. When the environment can cause the galloping of the power transmission line, the early warning operation is started, and the galloping degree of thepower transmission line is determined through the comparison of the motion track and the image information. The system can achieve the automatic judgment of whether a meteorological environment can cause the galloping of the power transmission line or not, can achieve the early warning timely, and determines the galloping condition through the comparison of the motion track and the image information. The monitoring result is more reliable.