An on-line monitoring device for lightning current of an overhead transmission line is composed of an Msp430 single chip microcomputer, an FPGA, a trigger circuit, an AD collection and matching circuit, an AD converter, a GPS module, a GPRS communication module, a solar panel, a charge controller, a storage battery, Rogowski coils, a digit circuit power supply and an analog circuit power supply, is installed at a cross arm of a pole tower, and is powered by solar batteries, and sensors are installed on an insulator chain of the pole tower of the power transmission line and a lightning conductor of the pole tower. When a placed near a monitored point is struck by lightning, the Rogowski coils output induction voltage. When the voltage reaches a certain threshold value, the trigger circuit of the device starts to collect lightning current signals, and sends a measurement result to a terminal computer through a wireless communication method. A lightning current oscillogram and waveform parameters of each path are obtained through comprehensive analysis of the terminal computer, and the pole tower is guided to carry out maintenance work.