A single-stage switching power supply includes a transformer, a voltage level generation circuit, a first switching circuit, a second switching circuit, a rectifying and filtering circuit, a feedback circuit and a control circuit. The rectifying and filtering circuit is connected to the secondary winding assembly and the system circuit for generating an output voltage. The feedback circuit generates a feedback signal in response to the output voltage. In response to the feedback signal and an operating-status signal issued by the system circuit, the first and second switching circuits are alternately enabled under control of the control circuit such that electric energy of a first DC voltage is transmitted from the first primary winding assembly to the secondary winding assembly. The first switching circuit is disabled but the second switching circuit is enabled under control of the control circuit when the operating-status signal is at a standby operating status.