The single-ended flyback converter, owing to its simple structure and electrical isolation characteristics, is widely used in various small-to-medium power auxiliary power supply systems. However, with the development of technologies such as magnetic confinement fusion and nuclear magnetic resonance, the steady-state stray magnetic fields around such devices can severely interfere with power supplies. To analyze the interference effects of steady-state magnetic fields on flyback converters, this study establishes a circuit model of a saturated transformer based on a mutual inductance equivalent model, constructs a flyback converter model under steady-state magnetic fields, investigates the influence of magnetic fields on the converter, and proposes magnetic interference mitigation measures for the converter. Finally, experimental verification is conducted. The results demonstrate that steady-state magnetic fields significantly reduce the magnetizing inductance of the transformer, thereby increasing the current rising rate and peak current, and deteriorating the output voltage ripple. When the magnetic flux density exceeds 40 mT, the output voltage experiences a severe drop. By optimizing the magnetic core structure and increasing the gap width, the converter maintains rated output under a 40 mT magnetic field. This results in a 46.2% reduction in primary current peak and a 44.9% decrease in output voltage ripple, effectively enhancing the magnetic immunity of the flyback power supply.