The development of multifunctional electromagnetic wave-absorbing materials capable of operating in complex environments has become critically important in the field of electromagnetic protection. However, achieving materials that combine high absorption efficiency with exceptional thermal stability remains a significant challenge. Owing to their lightweight hollow structure, low thermal conductivity, and high thermal stability, hollow SiC/C fibers present a promising solution to the problem of high-temperature electromagnetic wave absorption. In this study, hollow SiC/C nanofibers were successfully synthesized via a combined hydrothermal and carbothermal reduction approach. The resulting hollow SiC/C nanofibers exhibit ultralight characteristics, high-temperature stability, good elasticity and fatigue resistance, and exceptional electromagnetic wave absorption performance, including an effective absorption bandwidth of 7.0 GHz at a thickness of 2.4 mm and an optimal reflection loss of −63.5 dB at a thickness of 1.4 mm. Moreover, the material demonstrates remarkable high-temperature dielectric stability, with its complex permittivity remaining virtually unchanged at 600 °C. The formulated strategy provides a feasible approach for designing SiC matrix composites with stable dielectric properties and efficient electromagnetic wave absorption.