The invention provides a variable-frequency power supply device. The variable-frequency power supply device comprises a transformer, a frequency conversion pulse width modulation circuit, an first isolation circuit, a rectifying and filtering circuit and a switching circuit, wherein the transformer is provided with a primary side and a secondary side; the frequency conversion pulse width modulation circuit is arranged on the secondary side of the transformer, and the frequency conversion pulse width modulation circuit is internally provided with a frequency conversion circuit, a pulse width modulation control circuit, a shielding control circuit and a protection circuit, wherein a detection circuit, a proportional control circuit and an oscillating circuit are arranged in the frequency conversion circuit; one end of the pulse width modulation control circuit is connected to the first isolation circuit; the rectifying and filtering circuit is arranged between the sampling circuit and the secondary side of the transformer; and the switching circuit is arranged between the primary side of the transformer and the first isolation circuit, the oscillation signal frequency of the frequency conversion circuit can be adjusted according to the load condition proportion, so that the pulse signal of the pulse width modulation control circuit reaches a variable frequency, and then the switching loss of the switching circuit and the rectifying and filtering circuit can be greatly reduced.