The invention discloses an online measurement system for a crossing span distance of an overhead transmission conductor, belongs to the technical field of power measurement, and aims at solving a problem that a conventional manual regular measurement method consumes a large amount of manpower and a large number of material resources. The system comprises an online measurement device for the crossing span distance of the overhead transmission conductor, an online measurement distance data communicator for the crossing span distance of the overhead transmission conductor, and an early warning device for the crossing span distance of the overhead transmission conductor. The data monitored by the online measurement device for the crossing span distance of the overhead transmission conductor istransmitted to the early warning device for the crossing span distance of the overhead transmission conductor through the online measurement distance data communicator for the crossing span distanceof the overhead transmission conductor, thereby achieving the real-time monitoring, transmission and early warning. The system achieves the online monitoring of the crossing span distance of the conductor in a state of windage yaw and other states, and achieves the real-time transmission of distance state data. Compared with conventional discrete point manual data collection and subsequent analysis, the system achieves the real-time monitoring, analysis and alarm, avoids the burden of manual data collection, and improves the line maintenance efficiency.