It is still a challenge to design and synthesize ferrite-based composites with strong absorption capability and broad frequency bandwidth. In this work, firstly, the binary SmFe5O12/CoFe2O4 composites with different mass ratio of SmFe5O12 to CoFe2O4 are synthesized by a facile hydrothermal process. Next, adding 10 mg graphite oxide (GO) into SmFe5O12/CoFe2O4 solution, series of novel reduced graphene oxide (rGO)/SmFe5O12/CoFe2O4 ternary nanocomposites are fabricated using a one-pot hydrothermal technique. The microstructure, morphology, chemical composition and magnetic property of as-synthesized nanocomposites are characterized by XRD, Raman, TEM, SEM, XPS and VSM. The magnetic performance of SmFe5O12 changes with the addition of CoFe2O4 and rGO. The microwave absorption measurement is carried out by vector network analyzer, where the as-synthesized samples exhibit excellent and tunable microwave adsorption performance. Herein, introducing CoFe2O4 and rGO into SmFe5O12 can effectively tune its dielectric and magnetic loss. Encouragingly, when the mass ratio of SmFe5O12 to CoFe2O4 is 1:3, the rGO/SmFe5O12/CoFe2O4 ternary nanocomposite possesses the maximum reflection loss (RL) of - 73.71 dB at 14.88 GHz with a thickness of 2.09 mm. The effective bandwidth (RL < -10 dB) of the as-synthesized sample is 7.12 GHz (from 10.80 to 17.92 GHz) with a thickness of 2.18 mm, which is higher than that of most microwave absorbers reported previously. Noticeably, the superior microwave adsorption performance of as-synthesized samples can be mainly attributed to the synergistic effect of dipole polarization, interfacial polarization, natural resonance, eddy current loss, multiple scattering and reflection. These results show that the as-synthesized rGO/SmFe5O12/CoFe2O4 ternary composites are promising candidates for the application in microwave adsorption with high reflection loss, broad bandwidth, lightweight and thin thickness.