The invention discloses a secondary voltage drop/secondary load on-line monitoring device and system and a monitoring method thereof. The on-line monitoring device comprises a first communication unit which is used for receiving measurement data, a microprocessor which is used for calculation processing and overall control, a multipath selection unit which is used for selecting access signals which need measurement and sending the access signals to a signal conditioning unit, the signal conditioning unit which is used for conditioning the access signals and carrying out sending to a first frequency multiplication unit, the first frequency multiplication unit which is used for carrying out frequency multiplication on conditioned signals and sending the conditioned signals after frequency multiplication to an analog-to-digital conversion unit, the analog-to-digital conversion unit which is used for sampling the signals to acquire sampling signals, a second communication unit which is used for receiving a control instruction of a front-end processor and sending test data to the front-end processor and a keyboard input unit which is used for inputting a field control instruction into the microprocessor. According to the invention, real-time on-line monitoring can be carried out on load, error and other main parameters of a metering device; manual field measurement is not needed; the workload is reduced; and the work efficiency is improved.