Copper (Cu)-containing single-source precursors (SSPs) for the preparation of SiCuCN-based ceramic nanocomposites were successfully synthesized for the first time using polysilazane (PSZ), copper(II) acetate monohydrate (CuAc), and 2-aminoethanol via nucleophilic substitution reactions at silicon (Si) centers of PSZ. The synthesis process, polymer-to-ceramic transformation, and high-temperature microstructural evolution of the prepared ceramics were characterized. Dielectric properties and electromagnetic wave (EMW) absorbing performance of the ceramics were investigated as well. The results show that the polymer-to-ceramic transformation finishes at ca. 900 ℃, and Cu nanoparticles are homogeneously distributed in a SiCN matrix, forming a SiCN/Cu nanocomposite. After annealing at 1200 ℃, the Cu nanoparticles completely transform into copper silicide (Cu₃Si). Interestingly, the thermal stability of the Cu nanoparticles can be strongly improved by increasing the free carbon content, so that a part of metallic Cu nanoparticles can be detected in the ceramics annealed even at 1300 ℃, forming a SiCN/Cu/Cu₃Si/C nanocomposite. Compared with SiCN, the SiCuCN-based nanocomposites exhibit strongly enhanced dielectric properties, which results in outstanding EMW absorbing performance. The minimum reflection coefficient (RC_min) of the SiCN/Cu/Cu₃Si/C nanocomposites annealed at 1300 ℃ achieves −59.85 dB with a sample thickness of 1.55 mm, and the effective absorption bandwidth (EAB) broadens to 5.55 GHz at 1.45 mm. The enhanced EMW absorbing performance can be attributed to an in situ formed unique network, which was constructed with Cu and Cu₃Si nanoparticles connected by ring-like carbon ribbons within the SiCN matrix.