The invention relates to a large cross section lead sag online monitoring system. The system comprises a signal acquisition device, a monitoring host, a power supply device, an information transmission device, and a monitoring center. The acquisition device comprises a current sensor, a double-shaft inclination angle sensor and a temperature sensor. The current sensor includes a first connector, a second connector, a second shell and a first shell which are coaxially and successively arranged from inside to outside. The monitoring system provided in the invention can collect scenes in real time, and determines whether to start circuit protection according to a collected lead current condition. A server predicts a lead sag change trend based on collected real-time data, which is convenient for power grid dispatching and management personnel to dynamically adjust a thermal stability load of a power transmission line in real time, a power grid operation capability is increased and safety of an operating line and crossed equipment is ensured. The safety of the lead sag online monitoring system of the invention is ensured.