The ingenious multicomponent microstructure design provides a suitable strategy for gaining high-performance multi-functional integrated materials. Herein, the thermally stable SiC/Si3N4 composite ceramic felt was successfully fabricated via two-step carbothermal reduction processes for advanced electromagnetic (EM) wave absorption properties with strong absorption and a broad effective absorption bandwidth. X-ray absorption near-edge structure (XANES) at the C K-edge and N K-edge of as-prepared samples were investigated to understand the conversion process and electronic structures. The abundant heterogeneous interfaces and complex morphology improved the superior absorption performance by forming excellent impedance matching, multiple interfacial polarization and dipolar polarization generated from the synergistic interaction of SiC and Si3N4 dual-phase felt. The prepared SiC/Si3N4 composite ceramic felt exhibited high absorption efficiency with a minimal reflection loss of –50 dB (exceeding 99.99% wave absorption) and wide absorption band (covering 7 GHz) with a thin thickness of 2.1 mm. Furthermore, the excellent structural robustness and resilient compressibility inherited from C felt, as well as the thermostability until 950 °C under O2 guarantee the stable and durable application of the SiC/Si3N4 composite ceramic felt to resist EM absorbers, especially for in extreme high-temperature environments.