Abstract: Electromagnetic wave (EMW)-absorbing materials have considerable capacity in the military field and the prevention of EMW radiation from harming human health. However, obtaining lightweight, high-performance, and broadband EMW-absorbing material remains an overwhelming challenge. Creating dielectric/magnetic composites with customized structures is a strategy with great promise for the development of high-performance EMW-absorbing materials. Using layered double hydroxides as the precursors of bimetallic alloys and combining them with porous biomass-derived carbon materials is a potential way for constructing multi-interface heterostructures as efficient EMW-absorbing materials because they have synergistic losses, low costs, abundant resources, and light weights. Here, FeNi alloy nanosheet array/Lycopodium spore-derived carbon (FeNi/LSC) was prepared through a simple hydrothermal and carbonization method. FeNi/LSC presents ideal EMW-absorbing performance by benefiting from the FeNi alloy nanosheet array, sponge-like structure, capability for impedance matching, and improved dielectric/magnetic losses. As expected, FeNi/LSC exhibited the minimum reflection loss of −58.3 dB at 1.5 mm with 20wt% filler content and a widely effective absorption bandwidth of 4.92 GHz. FeNi/LSC composites with effective EMW-absorbing performance provide new insights into the customization of biomass-derived composites as high-performance and lightweight broadband EMW-absorbing materials.