The present disclosure relates to a supply voltage generating circuit and a switching power supply having the circuit. The supply voltage generating circuit provides a supply voltage for a power terminal of a control circuit in the switching power supply. The supply voltage generating circuit includes a switching circuit, a unidirectional conduction circuit and an energy storage circuit. In a time period during which a main power transistor is turned off, the switching circuit is turned off and a rectifying device provides a current to the energy storage circuit through the unidirectional conduction circuit, in a case that the supply voltage is smaller than a predetermined voltage reference; and the switching circuit is turned on and the current output from the rectifying device flows to the switching circuit, in a case that the supply voltage is larger than or equal to the predetermined voltage reference. When the supply voltage generating circuit is used in a switching power supply, the control circuit has improved controllability for supplying electric energy, and the switching power supply has an increased integration level and a simplified circuit structure, and will not be adversely influenced by resonance.