The invention relates to an ultra-low frequency alternating-current (AC) constant-current demagnetization device of a large transformer or mutual inductor, comprising a display (1) and a CPU control unit (2), wherein the CPU control unit (2) is connected with a power-frequency signal generator (3); the power-frequency signal generator (3) is connected with a voltage power amplifier (4); the voltage power amplifier (4) is connected with a voltage booster (5); the CPU control unit (2) is also connected with an ultra-low frequency signal generator (6); a switch is arranged between a transformer or a mutual inductor (8) to be tested and the voltage booster (5); and a switch is arranged between the transformer or the mutual inductor (8) to be tested and a current power amplifier (7). The invention overcomes the defects of long demagnetization time, easy transformer coil deformation, and the like of the traditional method and has the advantages of short demagnetization time, light equipmentweight, and the like. The invention also discloses a demagnetization detection method adopting the demagnetization device.