The application of electromagnetic waves has reached every aspect of human life, but the search for superior electromagnetic wave absorbent materials has been a constant quest of researchers. The application of heterogeneous structures has been favored by researchers of electromagnetic wave absorbent materials and the quest for simple preparation methods and homogeneous distribution of heterogeneous structures is continuing. In this study, we synthesized carbon sphere/Fe3O4 nanocrystal (CS/Fe3O4) composites by uniformly decorating Fe3O4 nanoparticles on the surface of carbon spheres through a simple strategy of expanding the heterogeneous structured interface. The heterogeneous interface formed by graphite and amorphous carbon in the carbon spheres is a boundary-type defect and combined with the magnetic loss capability of the Fe3O4 nanocrystals, this composite material has excellent electromagnetic wave absorption properties. The composite material synthesized with 0.05 M solution of iron nitrate has the best electromagnetic wave absorption performance of all samples due to the synergistic effect of interfacial polarization, eddy current loss, defect engineering, and magnetic energy attenuation capability. Reflection losses of -50.932 dB and -49.143 dB were achieved at 4.65 GHz and 10.6 GHz respectively, corresponding to thicknesses of 3.74 mm and 1.74 mm. In addition, the widest effective absorption bandwidth (EAB) at 1.27 mm was 4.5 GHz (13.50-18 GHz). This study enhances the electromagnetic wave absorption performance of carbon spheres by surface-decorating Fe3O4 nanoparticles, solves the problem of homogeneity of decorative magnetic oxides on the surface of carbon-based materials, and provides new ideas for the design of controllable, lightweight, ultra-thin composites of carbon-based electromagnetic wave absorbent materials that possess strong electromagnetic wave absorption capability.