Ba_0.9La_0.1Fe_11.9Ni_0.1O₁₉ nanoparticles were prepared by a co-precipitation method, and Ba_0.9La_0.1Fe_11.9Ni_0.1O₁₉/reduced graphene oxide/polyaniline ternary nanocomposites (BF/RGO/PANI) were prepared by in situ polymerization. Microwave reflection loss, and the permittivity and permeability of BF/RGO/PANI nanocomposites were investigated in the frequency range 2–18 GHz. The prepared samples possessed excellent electromagnetic wave-absorbing performances, and the optimal impedance matching of BF/RGO/PANI nanocomposites was found. The minimum reflection loss reached −49.1 dB near the frequency of 14.08 GHz with a thickness of 1.9 mm and its absorption bandwidth with a reflection loss below −10 dB ranged from 12.4–16.72 GHz with a bandwidth of 4.32 GHz. Our study illustrates that the as-prepared BF/RGO/PANI nanocomposites are promising candidates for use in electromagnetic wave-absorbing materials.