SiOC-based ceramics are considered promising electromagnetic wave-absorbing materials because of their lightweight, high-temperature resistance, and heat insulation properties. Herein, SiOC@C ceramic nanospheres were prepared using a liquid phase method combined with the polymer-derived ceramics (PDCs) method, followed by heat treatment in N₂ and Ar atmospheres at different temperatures. The morphology, microstructure, phase composition, and electromagnetic wave absorption performance of the SiOC@C ceramic nanospheres were investigated in detail. The SiOC@C ceramic nanospheres obtained in the Ar atmosphere showed an RL_min of -67.03 dB; whereas the SiOC@C ceramic nanospheres obtained in the N₂ atmosphere exhibited an RL_min value of -63.76 dB. The outstanding electromagnetic wave absorption performance of the SiOC@C ceramic nanospheres was attributed to the synergistic effect between conductive loss, interfacial/defect polarization loss, multiple reflections and scattering. Therefore, this research provides valuable insights into the design and fabrication of SiOC ceramic-based electromagnetic wave absorbers.