Abstract: Thin-layered flaky high-purity CoNi/C materials were prepared by a simple hydrothermal method and the effect of carbon content on the wave-absorbing properties of CoNi/C composites was investigated. The results indicate that CoNi/C primarily exhibits a flake-like morphology, accompanied by a small fraction of block-shaped particles. With increasing carbon content, the proportion of CoNi/C lamellae first increases and then decreases. The samples with excellent wave-absorbing properties were best at a carbon content of 2.4 mmol. At a thickness of 1.8 mm, the effective absorption bandwidth reaches 5.60 GHz, and an optimal reflection loss of −45.28 dB is achieved at 10.88 GHz.The excellent wave-absorbing ability of CoNi/C composites is attributed to the conductive network formed by the thin-layer lamellar morphology of the samples that improves the loss of the electromagnetic wave and optimizes the impedance matching characteristics and attenuation characteristics of the materials. This work provides a new idea for the preparation of thin-layer sheet-like wave-absorbing materials.