With the rapid development of new aerospace vehicles, there are increasing demands for higher structural reliability and wideband microwave stealth requirements for the components operating under high-temperature condition. SiBCN based metastable ceramics exhibit good resistance to high temperature, thermal shock, ablation, long-term oxidation, and creep, showcasing great potential in the field of high-temperature structural microwave absorption. However, their ability to absorb electromagnetic waves is limited by low dielectric loss. In this study, the SiBCN-rGO ceramic fibers with good mechanical and microwave-absorbing properties were prepared using the wet spinning technology. Results showed that the as-prepared SiBCN-rGO ceramic fibers possessed porous structure, with porosity increasing with the increase of reduced graphene oxide (rGO) content. Additionally, both high rGO content and high fiber specific surface area promoted the crystallization of SiC within the amorphous matrix. The introduction of rGO significantly enhanced the tensile properties of the resulting ceramic fibers. As the mass fraction of rGO increased from 0 to 4%, the fibers’ elongation at break increased from 8.05% to 18.05%, and the tensile strength increased from 1.62 cN/dtex (0.324 GPa) to 2.32 cN/dtex (0.464 GPa). The increase of rGO content also reduced the electrical resistivity of the ceramic fibers. Moreover, as the rGO mass fraction increased from 0 to 4%, both the real and imaginary parts of the fibers’ dielectric constant decreased, while the loss tangent gradually increased. The SiBCN-rGO ceramic fibers with those containing 6% (mass fraction) rGO exhibited excellent wave-absorption performance, showing the minimum reflection coefficient of -50.90 dB at 9.20 GHz and an effective absorption bandwidth of 2.3 GHz, indicating promising applications in wave-absorbing ceramic matrix composites.