A control circuit can include: an output current sampling circuit configured to sample a current through a power switch of a switching power supply operating in a quasi-resonant mode, and to generate an output current feedback signal that represents an output current of the switching power supply; and a valley locking circuit configured to receive the output current feedback signal and a plurality of valley switching threshold voltages, where the valley locking circuit is switched to a corresponding valley from a valley currently being locked when the output current feedback signal is increased or reduced to one of the plurality of valley switching threshold voltages.