Nowadays, high-performance electromagnetic wave (EMW) absorption materials derived from low-cost biomass materials have attracted widespread attention. Herein, a novel biomass‑carbon with porous array structures derived from waste soybean dregs is prepared by a hydrothermal method combined with heat treatment using formaldehyde and KOH as activators. The as-prepared biomass‑carbon with significantly enhanced specific surface area can possess more charge transmission paths, increased multiple reflections and scattering, multiple loss modes, and improved impedance matching, leading to the improvement of EMW absorption performances. The obtained single-component biomass‑carbon without any functional fillers exhibits an effective EMW absorption capability with a wide absorption bandwidth of 4.8 GHz at a thickness of 3.5 mm. This study will provide a valuable approach for the preparation of biomass‑carbon based EMW absorption materials with controllable porous structures by the activation process. Display omitted •High-performance single-component biomass carbon derived from soybean dregs•Unique porous array structures formed through an activation process.•Effective EMW absorption capability with a wide absorption bandwidth of 4.8 GHz.