The invention discloses a control circuit and a control device for a flyback power factor corrector. The control circuit and the control device are used for controlling a flyback converter which works in a critical continuous mode to realize unit power factor; breakover time of a flyback converter circuit which works in the critical continuous mode is controlled by the control circuit so that breakover time of a switch tube varies with input voltage and output voltage in a power frequency period; turn-off time of the switch tube is determined by a zero-passing moment of transformer secondary side current; and after energy of the transformer is totally released, and because a parasitic junction capacitor of the switch tube and an excitation inductor produce resonance, the switch tube is turned on by detecting a secondary side current zero-passing moment of an auxiliary winding. By the method, a power factor of the conventional flyback power factor corrector in the critical continuous mode can be improved, and the unit power factor of the flyback converter in the critical continuous mode can be acquired. The unit power factor flyback power factor corrector in the critical continuous mode is high in efficiency, and meanwhile, the unit power factor can be acquired in the whole range of the input voltage.