Novel porous flower-like NiO@graphene composites were prepared using a method involving a facile hydrothermal reaction and an annealing process. The precursor Ni(OH)₂ was grown to a flower-like microsphere under weak basic conditions and was partly coated with graphene oxide flakes. The final porous composites were obtained after the annealing process. The structure of the flower-like NiO@graphene composites was characterized by XRD, Raman spectroscopy, XPS, SEM, TEM, and N₂ adsorption–desorption. The influence of base strength on the morphology of the three-dimensional structure of NiO@graphene was investigated. The flower-like NiO@graphene is highly porous and has a large surface area of 107 m² g⁻¹. As an absorber, the composite with a filler loading of 25 wt% exhibited superior microwave absorption capacities owing to its special porous flower-like structure, polarization effect, good impedance matching, and synergistic action. The maximum reflection loss can reach −59.6 dB at 14.16 GHz, and the absorption bandwidths (RL below −10 dB) ranged from 12.48 GHz to 16.72 GHz with a thickness of only 1.7 mm. The results indicate that the lightweight NiO@graphene composites with high-performance microwave absorption properties are promising materials for Ku-band electromagnetic wave absorption.