The invention discloses a switching power supply capable of reducing a low-frequency secondary-side common-mode voltage. The power supply mainly has two common-mode filtering circuit devices to prevent an AC voltage and an AC current from coupling to a secondary side of a transformer. Thus, the electrical safety and the quality in use of the associated electronic apparatus can be enhanced, a potential difference between the electronic apparatus and the ground may be greatly reduced, and an AC hum of a speaker coupled to an output of the power supply may be reduced or even eliminated.