Abstract: Core shell Fe3O4-polypyrrole composite microspheres were prepared by one-step etching method， and their structures and electromagnetic parameters were characterized by FT-IR， XRD， TG and vector network analyzer. The results showed that the Fe3+ produced by hydrochloric acid etching initiated the polymerization of pyrrole on the surface of Fe3O4 microspheres. The core-shell structure was confirmed by scanning electron microscopy and transmission electron microscopy. The formation of polypyrrole was confirmed by FT-IR spectra. The XRD spectra showed that the crystal shape of the nuclei did not change during the reaction. The thermo-gravimetric curve and hysteresis regression lines showed that the content of Fe3O4 decreased and the content of polypyrrole increased with the increase of reaction time. When the reaction time was 5 minutes， the sample had the best microwave absorbing performance， the lowest reflection loss was -41.9 dB， and the effective absorption bandwidth was 6 GHz.The electromagnetic shielding effectiveness of Fe3O4-polypyrrole based coating was ≥36 dB in the 30 MH～1 GHz band.