The present invention provides a switching control circuit having a valley voltage detector to achieve the soft switching and improve the efficiency of a power converter. The switching control circuit includes a control circuit coupled to the feedback signal to generate a switching signal. Through an output circuit, the switching signal drives a switching device for switching a transformer and regulating the output of the power converter. The valley voltage detector is coupled to an auxiliary winding of the transformer for generating a control signal in response to the voltage of the transformer. The control signal is used for enabling the switching signal. The switching signal further turns on the switching device in response to a valley voltage across the switching device.