The work aims to explore the effects of different nano-structures on the microwave absorption properties of Fe3O4 nanomaterials. The magnetic Fe3O4 nanomaterials with the morphologies of nanospindles, nanotubes and opened-hollow spheres were obtained by hydrothermal method and carbothermal reduction method. The phase, morphology and electromagnetic properties of the materials were tested by XRD, SEM and vector network analyzer. Among the Fe3O4 composites with three morphologies, opened-hollow spheres exhibited the smallest dielectric constant and the highest impedance matching coefficient, and its magnetic loss exhibited two resonance peaks, and the attenuation coefficient of the Fe3O4 opened-hollow spheres was significantly higher than the other two Fe3O4 nanomaterials in the frequency range of 7.5~14.3 GHz, which were beneficial for widening the microwave absorption bandwidth. The reflection loss bandwidth of the Fe3O4 opened-hollow spheres was larger than the other two structures in the thickness range of 1.8~3.0 mm, achieving a qualified absorption bandwidth of 5.0 GHz (90% absorption) covering from 7.3 to 12.3 GHz at the thickness of 2.2 mm. The double magnetic resonances of the Fe3O4 opened-hollow spheres enhance the microwave attenuation and impedance matching characteristics of the material. The microwave absorption capacity of the opened-hollow spheres is significantly better than nanospindles and nanotubes. Through the design of superstructure, the microwave absorption by bandwidth can be realized.