Abstract: High-entropy design is attracting growing interest as it offers unique structures and unprecedented application potential for materials. In this article, a novel high-entropy ferrite (CoNi)_x/2(CuZnAl)_(1−x)/3Fe₂O₄ (x = 0.25, 0.34, 0.40, 0.50) with a single spinel phase of space group Fd\bar3m was successfully developed by the solid-state reaction method. By tuning the Co–Ni content, the magnetic properties of the material, especially the coercivity, changed regularly, and the microwave absorption properties were improved. In particular, the effective absorption bandwidth of the material increased from 4.8 to 7.2 GHz, and the matched thickness decreased from 3.9 to 2.3 mm, while the minimum reflection loss remained below −20 dB. This study provides a practical method for modifying the properties of ferrites used to absorb electromagnetic waves.