Carbon-based magnetic composites are promising alternatives to pure magnetic nanoparticles (MO_x, M = Fe, Co, Ni) and spinel ferrites (MFe₂O₄, M = Fe, Co, Ni, Mn, Zn) as electromagnetic (EM) wave absorption materials, which however suffer from complex processes and have poor microwave absorption properties. Herein, we anchored Fe₃O₄ porous spheres onto carbon nanotubes (CNTs) via a simple solvothermal method. The formed Fe₃O₄/CNTs nanocomposites show a three-dimension (3D) network. The improved effective bandwidth (3.9 GHz) and, more importantly, remarkable EM microwave absorption performances (−51 dB at 5.52 GHz) are observed in 3D Fe₃O₄/5 wt% CNTs nanocomposites. The enhanced microwave absorption performances are attributed to the high surface areas and porous structure of magnetic Fe₃O₄ spheres, which presented a good synergetic role with CNTs. Furthermore, the controllable permittivity in nanocomposites was developed by adjusting the CNT content, which balanced the permeability to obtain a good impedance matching. This work demonstrates a simple approach to enhancing the microwave absorption performances of EM wave absorption materials.