A control circuit for a switching power supply (SPS) includes power input and output interfaces, a relay, a relay driving circuit, a microprocessor, and an alternating current/directing current (AC/DC) converter. The power input interface receives an external alternating current (AC) power signal, and transmits the AC power signal to the SPS via the power output interface. The AC/DC converter transforms the AC power signal into a direct current (DC) power signal to supply for the relay, the relay driving circuit, and the microprocessor. When a computer is turned on, the microprocessor sends a first control signal to control the relay driving circuit to drive the relay to connect the power input and output interfaces. When the computer is turned off, the microprocessor sends a second control signal to control the relay driving circuit to drive the relay to cut off connection between the power input and output interfaces.