Nowadays, constructing a high-efficiency microwave absorber with strong absorption strength and wide absorption bandwidth is urgently demanded for practical applications in the microwave absorption (MA) field. In this work, the Co/MnO@C nanocapsules with magnetic/dielectric dual cores were fabricated through in-situ carbon reduction of the MnCo2O4 nanoparticles. The effects of calcination temperature on electromagnetic parameters and MA ability are investigated in detail. Notably, the sample obtained at 900 ℃ exhibits the minimum reflection loss (RL) value of - 97.5 dB at 10.6 GHz, while the effective absorption bandwidth (EAB, RL ≤ - 10 dB) is up to 8.1 GHz (9.9–18 GHz) at 2.1 mm thickness, covering half of X-band and whole Ku-band. The excellent MA performance is attributed to the core–shell structure, multiple heterogeneous interface polarizations, synergistic effects between magnetic and dielectric components, as well as promoted impedance matching. Importantly, the electric and magnetic field distributions are simulated by ANSYS software, and the polarization and magnetic coupling behaviors are deeply analyzed. The simulated radar cross-section (RCS) results confirm that the optimized Co/MnO@C nanocapsules can efficaciously attenuate more electromagnetic energy in the practical environment. It is believed that the as-prepared Co/MnO@C nanocapsules are considered to be promising high-efficiency microwave absorbers for practical applications.