Abstract: Discarded walnut shells were modified by the chemical composition of special steel slag ultrafine powder to obtain steel-slag-based biomass-activated carbon. The influences of the mass ratio of discarded walnut shell ultrafine powder and special steel slag ultrafine powder, the fineness of special steel slag ultrafine powder, and adsorption ambient temperature on the absorbed chlorine gas performance of steel-slag-based biomass-activated carbon were studied. Results show good chlorine gas absorption performance when the mass ratio of discarded walnut shell ultrafine powder and special steel slag ultrafine powder is 100∶6, the fineness of special steel slag ultrafine powder is 600 mesh, and adsorption ambient temperature is 30 ℃. The magnetic property of Fe₂O₃ in special steel slag ultrafine powder is conducive to the formation and enrichment of chlorine gas on the surface of steel-slag-based biomass-activated carbon, improving its absorption performance. Catalytic performance of CuO and MnO helps promote the absorbing performance of steel-slag-based biomass-activated carbon. When the fineness of special steel slag ultrafine powder is excessively large, agglomeration of small particle size occurs and affects the adsorption capacity of steel-slag-based biomass-activated carbon to chlorine gas. When the particle size of special steel slag ultrafine powder is small, the special steel slag ultrafine powder with good uniformity is less effective in improving the adsorption of chlorine on the steel-slag-based biomass-activated carbon. The higher adsorption ambient temperature may lead to the analytical phenomenon of chlorine gas from steel-slag-based biomass-activated carbon. Moreover, no superfine agglomeration and deposition of special steel slag ultrafine powder on the surface of steel-slag-based biomass-activated carbon are observed. The obtained carbon exhibits the layered structure characteristics and provides space for chlorine gas adsorption.