The invention discloses a startup circuit of a supply voltage for a power factor correction (PFC) circuit and a switching power supply using the same. The startup circuit of the supply voltage includes an alternating-current (AC) input voltage sampling terminal, a PFC output voltage sampling terminal, a supply output terminal and an external supply voltage input terminal, a comparator circuit and a switch circuit. The startup circuit of the supply voltage samples an AC input peak voltage through the AC input voltage sampling terminal, samples a voltage at a positive terminal of a filter capacitor coupled to an output terminal of the PFC circuit through the PFC output voltage sampling terminal, and compares them to control whether or not the supply voltage received from the external supply voltage input terminal is transferred to the supply output terminal coupled to a control chip of the PFC circuit. The invention can prevent an inductor of the PFC circuit from saturation conducting under the condition of fast on/off so as to protect internal electric components of PFC circuit from damage.