This paper, taking FeCl3·6H2O and CH3COONa as source materials, prepares Fe3O4 powders by hydrothermal method to compare the superior of absorption with solid Fe3O4 powders. The paper analyzes the crystal structure of Fe3O4 powders by X-ray diffraction (XRD), and the size and the morphology of the prepared products by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The paper tests the electromagnetic parameters of coaxial samples by network analyzer to obtain the microwave absorption of porous Fe3O4 powders. The results indicate that the density of porous Fe3O4 powders is lower than that of the solid Fe3O4 powders, and the real part of permittivity of porous Fe3O4 powders is absolutely higher. When the thickness of samples is thicker than 4 mm, the absorption of prepared porous Fe3O4 powders is definitely superior.