Abstract: Lightweight and efficient carbon-based microwave absorbents are significant in addressing the increasing severity of electromagnetic pollution. In this study, hierarchical NiO/Ni nanosheets with a tuneable phase and morphology supported on a carbon fiber substrate (CF@NiO/Ni) were fabricated using a hydrothermal approach and post-annealing treatment. As the annealing temperature increases, more metallic Ni is formed, and an apparent porosity appears on the sheet surface. Benefiting from the advantages of a three-dimensional (3D) conducting network, hierarchical porous structure, reinforced dipole/interface polarization, multiple scattering, and good impedance matching, the CF@NiO/Ni-500 composite exhibits an excellent microwave absorption performance even at a filling rate of only 3wt%. Specifically, its minimal reflection loss is −43.92 dB, and the qualified bandwidth is up to 5.64 GHz. In addition, the low radar cross-section area of the CF@NiO/Ni composite coating confirms its strong ability to suppress electromagnetic wave scattering. We expect that this work could contribute to a deeper understanding of the phase and morphology evolution in enhancing microwave absorption.