An on-line monitoring device for a medium-voltage overhead line of a distribution network comprises a terminal and at least one probe. The composition and the connection relationship of the each probe are as follows: a current transformer, a signal processing circuit and a minimum CPU processing module are unidirectionally and electrically connected in sequence; a power module, a clock module and a communication module are bidirectionally and electrically connected with the CPU processing module respectively; and the signal processing circuit is unidirectionally and electrically connected with a pre-posed storage. The composition and the connection relationship of the terminal are as follows: an MESH communication module, a GSM communication module, a power module and a clock module are bidirectionally and electrically connected with a CPU module respectively; and a solar panel is connected with the power module. The average standby power consumption of the probes in the device is smaller than 40muA@3.6V, the average operating power consumption of the probes is less than 80muA@3.6V, the probe can get electricity to work at 5A current, and a communication device can work for 15 consecutive days in overcast conditions.