SiCnws/SiOC nanowires composites with a 3D network structure, exhibiting enhanced electromagnetic microwave absorption properties, were synthesized using carbothermal reduction. The solid precursor, organosilane waste, was employed, enabling cost-effective and environmentally friendly production. The structure and morphology of SiCnws/SiOC nanowires composites were extensively investigated by XRD, SEM, TEM, XPS spectra, and Raman spectra. Moreover, the composite demonstrated superior electromagnetic wave absorbing properties, including wide effective absorption bandwidth (EAB) of 2.73 GHz, strong minimum reflection loss (RLmin) of −52.66 dB, and thin thickness of 2.40 mm. These excellent electromagnetic wave attenuation properties stem from the synergistic strategy of dielectric and conductive losses, resulting from the unique 3D network heterostructural architecture. This work not only offers improved EMW absorption solutions but also contributes to waste recycling and resource recovery.