Large-quantity core-shell carbon spheres/Fe3O4 nanocomposites were synthesized via a simple solution method. The phase structures and morphologies of the composite had been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the carbon spheres were covered by a layer of Fe3O4 nanoparticles with a diameter of about 25 nm. Measurements of the electromagnetic parameters of the samples showed that microwave absorption properties of the carbon sphere-Fe3O4 nanocomposites were much better than that of the pure Fe3O4 or the mixtures of carbon spheres and Fe3O4 microspheres. The optimal reflection loss (RL) reached −37.8 dB at 14.8 GHz for a layer thickness of 2.0 mm, which is favorable for application of our samples in a wide frequency range.