Silicon carbide fiber (SiCf) has a promising potential to develop structural electromagnetic wave (EMW) absorption materials. Tuning dielectric properties of SiCf is necessary to obtain excellent EMW absorption performances, which is always realized by the introduction of extra dielectric loss phases. Herein, in-situ grown polycrystalline carbon nanowires (CNW) as novel dielectric fillers, possessing moderate conductivity as well as efficient loss ability, were used to improve the EMW absorption performance of SiCf. When the content of CNW is suited, CNW can interconnect with each other and form a multiporous network on the surface of SiCf. Such a unique network makes a great contribution to good impedance matching as well as strong conductivity loss and polarization loss, which endows SiCf with an effective absorption broadband (EAB) of 4.14 GHz at a thickness of 3.0 mm, demonstrating superior EMW absorption performance than most previously reported SiCf/carbon hybrids.