Herein, functionalized carbon nanospheres (CNs) are encapsulated within reduced graphene oxide (rGO) layers as a high-performance microwave-absorbing material. The obtained sandwich-like CNs@rGO nanocomposite is characterized by a well-matched characteristic impedance and multiple dielectric relaxation processes, resulting in a strong reflection loss (-33.09 dB at 14.01 GHz) and broad effective bandwidth of 12.2-17.6 GHz (5.4 GHz) with a low thickness of 2.0 mm, which is superior to many reported carbon absorbers and even some magnetic absorbers. This remarkable microwave absorption property is ascribed to the sandwich structure, indicating that the as-prepared sandwich-like CNs@rGO nanocomposites are potential candidates for lightweight microwave absorption materials.