The invention relates to a flyback switching power supply and a control circuit thereof. The control circuit comprises a pulse modulation controller which is used for outputting the PWM to drive a switch tube to be switched on and off. The comparison port of the pulse modulation controller is connected with the output end of an adjustment control module. The adjustment control module comprises a controllable precise voltage-stabilization source. The sampling end of the controllable precise voltage-stabilization source is used for being connected with an output voltage signal. The control circuit further comprises an overload protection module. The overload protection module comprises a first comparator. The non-inverting input end of the first comparator is used for being connected with anoutput current signal, and the inverting input end of the first comparator is connected with a first reference value. The output end of the first comparator is connected with the sampling end of thecontrollable precise voltage-stabilization source. According to the invention, the output voltage is controlled according to the change of the output voltage and the change of the overload current signal, so that the reliability is high. The dynamic response speed is increased, so that the switching frequency is accurately controlled. The duty ratio is adjusted.