The ternary composites of poly(3,4-ethylenedioxythiophene)-reduced graphene oxide–Co3O4 (PEDOT–RGO–Co3O4) were synthesized and the electromagnetic absorption property of the composites was investigated. The structure of the composites was characterized with Fourier-transform infrared spectra, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscope. The electromagnetic parameters indicate the enhanced electromagnetic absorption property of the composites was attributed to the better impedance matching. On the basis of the above characterization, an electromagnetic complementary theory was proposed to explain the impedance matching. It can be found that the maximum reflection loss of PEDOT–RGO–Co3O4 can reach −51.1 dB at 10.7 GHz, and the bandwidth exceeding −10 dB is 3.1 GHz with absorber thickness of 2.0 mm. Therefore, the PEDOT–RGO–Co3O4 composites, with such excellent electromagnetic absorption properties and wide absorption bandwidth, can be used as a new kind of candidate for microwave absorbing materials.