Porous Fe3O4-decorated graphene (GN–Fe3O4) composites with different microstructures were successfully synthesized by a modified two-step method. The microstructure and morphology were confirmed by X-ray diffraction (XRD), transmission electron microscopy and scanning electron microscopy. XRD studies show that the products consist of highly crystallized Fe3O4 but disorderedly stacked GN sheets. Electron microscopy images reveal that Fe3O4 nanoparticles with different sizes and microstructures are uniformly coated on both sides of GN sheets, without large vacancies or apparent aggregation. Electromagnetic wave absorption properties of epoxy containing 30 wt.% GN–Fe3O4 composites were investigated at room temperature in the frequency range of 0.5–18 GHz. In particular, the porous, flower-like Fe3O4-decorated GN sample exhibits an enhanced dielectric loss due to the porous microstructure of Fe3O4. The multiple absorbing mechanisms attribute to the improved impedance matching which indicates the as-prepared porous GN–Fe3O4 composites could be a potential candidate for lightweight electromagnetic wave absorption materials.