• Bi-phase high-entropy composites (BPHEC) with FeCoNiCr0.4Cu0.2 alloy and (FeCoNiCrCu)3O4 oxide are fabricated via low-temperature oxygen bath, enabling precise electromagnetic gene regulation.
• The BPHEC achieves ultra-broadband absorption (633 MHz at 5 mm thickness) and a minimum reflection loss of −12.8 dB, with RCS reduction up to 18.34 dB m2 in multilayer designs.
• Temperature-stable absorption (593–691 MHz) from −50 to 150 °C, along with high Curie temperature and oxidation resistance, ensures environmental adaptability.
• The study elucidates formation and regulation mechanisms, offering a tunable strategy for low-frequency EMW absorption materials.