Abstract: A new type of composite filler was designed by a modified sol–gel method using fly ash (FA), Fe(NO₃)₃∙9H₂O, and Ni(NO₃)₂∙6H₂O as raw materials. The composite filler was a spherical core–shell structure composed of FA as the core and NiFe₂O₄ as the shell. Further, the composite filler was added into the silicone rubber to fabricate the high temperature vulcanized microwave absorption materials; X-ray diffraction, fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscope confirmed that NiFe₂O₄ was successfully coated on the surface of FA and formed a uniform and continuous coating layer. As expected, silicone rubber filled with the composite filler had a minimum reflection loss of −23.8 dB at 17.5 GHz with the thickness of 1.8 mm, while the effective absorption bandwidth was as high as 12 GHz. The addition of the composite filler greatly enhanced the microwave absorption properties of the system, which was resulted from multiple losses mechanism: interface polarization losses, magnetic losses, and multiple reflection losses. Also, silicone rubber filled with the composite filler exhibited excellent thermal stability, flexibility, environmental resistance, and hydrophobicity compared with traditional silicone rubber. Therefore, this work not only responds to the green chemistry to achieve efficient FA recovery, but also devises a new strategy to prepare microwave absorption materials with strong potential for civilian applications.