The invention discloses a flyback switching power circuit. A valley filling circuit is adopted in a filter circuit of the flyback switching power circuit. The work voltage of a capacitor C1 and the work voltage of a capacitor C2 in the valley filling circuit are reduced by a half, -40 DEG C electrolytic capacitor can be adopted, and a third capacitor C3 is in parallel connection to a charging diode D3 in an existing valley filling circuit, so that the withstand voltage of the third capacitor C3 is half of a peak value of input pulse direct current, and high-frequency ripple current of a back switching power 300 can be fully and effectively absorbed. A varistor RV1 and a varistor RV2 are arranged before and behind a rectifier bridge 101, and a thermistor NTC, a safety capacitor X1 and an inductor L form a multi-level two-way filter and protection circuit. The flyback switching power circuit has the advantages that the power factor is high and not affected by a value of the C3, the topological structure of the circuit is simple, the absorption effect on surging voltages is good, cost is low, the size is small, and the difference mode conduction interference degree of the flyback switching power circuit reaches the CLASS B in GB9254.