Electromagnetic absorption (EMA) materials with light weight and harsh environmental robustness are highly desired and crucially important in the stealth of high-speed vehicles. However, meeting these two requirements is always a great challenge, which excluded the most attractive lightweight candidates, such as carbon-based materials. In this study, SiC_nw-reinforced SiCNO (SiC_nw/ SiCNO) composite aerogels were fabricated through the in-situ growth of SiC_nw in polymer-derived SiCNO ceramic aerogels by using catalyst-assisted microwave heating at ultra-low temperature and in short time. The phase composition, microstructure, and EMA property of the SiC_nw/SiCNO composite aerogels were systematically investigated. The results indicated that the morphology and phase composition of SiC_nw/SiCNO composite aerogels can be regulated easily by varying the microwave treatment temperature. The composite aerogels show excellent EMA property with minimum reflection loss of -23.9 dB@13.8 GHz, -26.5 dB@10.9 GHz, and -20.4 dB@14.5 GHz and the corresponding effective bandwidth of 5.2 GHz, 3.2 GHz, and 4.8 GHz at 2.0 mm thickness for microwave treatment at 600 ℃, 800 ℃, and 1000 ℃, respectively, which is much better than that of SiCN ceramic aerogels. The superior EMA performance is mainly attributed to the improved impedance matching, multi- reflection, multi-interfacial polarization, and micro current caused by migration of hopping electrons.