A flyback isolated high step-up DC/DC converter is proposed, which uses the ratio of a flyback transformer and a switched capacitor to improve the output voltage gain and meet the electrical isolation of input and output. With the introduction of the switched capacitor, the secondary-side windings of the flyback transformer charge the voltage doubled capacitor when the switch is off while supplying power to load when the switch is on, which improves the utilization rate of transformer windings and solves the resonance problem of output diode and leakage inductor of the flyback converter. Because of the leakage inductor of the transformer, all diodes are turned off with zero current, and there is no reverse recovery problem. An active-clamp circuit is adopted to realize the recovery and reuse of leakage inductor energy, suppress the voltage peak of the switch, and realize the zero-voltage-switching of the switch, thus improving the reliability and efficiency of the converter. Finally, the operating principle and performance characteristics of the proposed converter are analyzed. Compared with the traditional active-clamp flyback converter, the superiority of the proposed converter is verified, and the theoretical analysis is verified by the experimental results of a 500 W prototype.