The invention discloses a real-time on-line monitoring device for power-frequency electromagnetic data. The device comprises a main controller, a power-frequency electromagnetic data acquisition unit, a time synchronization unit, a data processing unit and a power supply system. The power-frequency electromagnetic data acquisition unit comprises a high-precision power-frequency electromagnetic sensor, a large-capacity battery, a power supply management module and at least two physical isolators, wherein, the power supply management module comprises a DC/DC (direct current) converter and a unilateral diode; the physical isolators are sequentially connected together, the input terminals of the physical isolators are connected with the power supply system, the output terminals of the physical isolators are connected with the anode of the unilateral diode, the cathode of the unilateral diode is connected with the DC/DC converter and the large-capacity battery, and the output terminal of the DC/DC converter is connected with the power input terminal of the high-precision power-frequency electromagnetic sensor; and the high-precision power-frequency electromagnetic sensor is also connected with the main controller and the large-capacity battery. By utilizing the real-time on-line monitoring device, long-time power supply and automatic charging of the high-precision power-frequency electromagnetic sensor can be realized, and influence of power line access on measuring results can be effectively shielded.