Spinel MgxCo1-xFe2O4 was prepared by hydrothermal method. The effects of the proportion of Mg and Co doping on the crystal structure, microstructure and absorption properties of magnesia cobalt ferrite were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and vector network analyzer (VNA). The electromagnetic wave absorption mechanism of magnesia cobalt ferrite was summarized. The results show that under the conditions of pH=10, crystallization temperature 180℃ and crystallization time 8 h, Mg-Co ferrite with irregular quadrilateral morphology was successfully prepared. When the ratio of Mg2+ and Co2+ is 5:5 and the thickness of the absorbing layer is 3.5mm, the reflection loss value at the frequency 9.50 GHz reaches -40.78 dB, and the effective absorption frequency band is 3.65 GHz (8.06~11.71 GHz), covering the X-band. The excellent absorbing properties are attributed to the combined action of natural resonance, exchange resonance and eddy current loss.