A power supply protecting circuit configured to protect an electronic device includes a normally open switch, a voltage dropping circuit, a first rectifier, and a relay having a coil and switch. A power terminal of the electronic device is connected to a power terminal of a power supply, the other power terminal of the electronic device is connected to the other power terminal of the power supply via the normally open switch. The power terminal of the electronic device is connected to the other power terminal of the electronic device via the voltage dropping circuit, the first rectifier, and the coil of the relay in turn. The switch of the relay is connected in parallel to the normally open switch.