A switching control circuit for a switching power converter is provided. The switching control circuit is coupled to a switching device and an auxiliary winding of a transformer. The switching control circuit includes a valley detecting circuit, a valley lock circuit, and a PWM circuit. The valley detecting circuit is coupled to receive a reflected voltage signal from the auxiliary winding of the transformer for outputting a control signal in response to the reflected voltage signal. The valley lock circuit is coupled to receive the control signal for outputting a judging signal in response to the control signal during a first period and a second period following the first period. The PWM circuit outputs a switching signal in response to the judging signal.