The invention relates to the technical field of overhead power transmission line operation state monitoring, and in particular to an online monitoring system of a power transmission line operation incident based on a magnetic flux sensor. The system herein includes on-site monitoring terminals which are installed on each tower and a backend server which is in wireless communication with the on-site monitoring terminals. Each on-site monitoring terminal includes a solar panel, a magnetic flux sensor, a GPS positioning module, an A/D converting module, a micro-processor, and a wireless communication module, wherein the solar panel, the GPS positioning module, and the A/D converting module are in separate connection to the micro-processor. The magnetic flux sensor is connected to the A/D converting module. The micro-sensor is in wireless communication to the backend server through the wireless communication module. According to the invention, the system is characterized by simple structure, easy operation and low investment, can simultaneously monitor and prevent mechanical operation fault and electrical operation fault of the overhead power transmission line, and increases the safety and reliability of the operation of power facilities.