A switching power supply with protection function includes a transistor controlled to be on or off by a PWM signal from a PWM controller to convert a DC voltage into a square wave signal, a filter filtering the square wave signal into an output voltage signal, the output voltage signal being fed back to the PWM controller to be compared with a predetermined voltage, and a switch device having an input terminal for receiving the output voltage signal from the filter, an output terminal for connecting to a load, and a control terminal. If the voltage of the output voltage signal from the filter is not equal to the predetermined output voltage, the PWM controller will not output a signal to the control terminal of the switch device and the switch device will be in an off state. The PWM controller adjusts the duty ratio of the PWM signal until the voltage of the output voltage signal from the filter is equal to the predetermined output voltage such that the PWM controller outputs a control signal to turn on the switch device to allow the load receiving the output voltage signal via the switch device.