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Under the background of carbon peak and neutrality, the composite microwave absorbents are prepared using coal‐based solid waste fly ash (FA) as raw material, supporting the implementation of comprehensive utilization for FA and the low‐cost preparation for microwave absorbents. CoNi‐loaded FA composites (CoNi@FA) are prepared via a simple rotary evaporation and sintering process, involving the introduction of Co2+–Ni2+ into FA as well as the subsequent reduction process during which the CoNi alloy transforms from the metallic oxide. As expected, the CoNi@FA800 manifests excellent microwave absorption performance, and the minimum reflection loss (RLmin) value is −39.8 dB. The effective absorption bandwidth (EAB) is extended to 3.4 GHz when the coating thickness is 2.0 mm. These are attributed to the introduction of CoNi alloy, endowing CoNi@FA composites with proper impedance matching and attenuation characteristics. Thus, the CoNi@FA will be a potential candidate in the field of microwave absorbent, which not only realizes the resource utilization of FA efficiently, but also makes a compromise between economic and environmental benefit simultaneously. [ABSTRACT FROM AUTHOR]