When chlorine leaks, measures should be taken immediately. Also the recovery and purification of chlorine-containing exhaust gas should be strengthened. At present, activated carbon with a developed porous structure and rich specific surface area is widely used to adsorb harmful gases. However the current production of activated carbon generally requires the consumption of natural resources such as wood and bamboo, resulting in high costs and is not conducive to sustainable development. Therefore, the preparation of activated carbon using biomass waste materials, and the modification of activated carbon by using metallurgical solid waste to reduce production and environmental costs further and improve adsorption properties, have become a hot spot in the field of activated carbon production. This study used special steel slag and discarded walnut shells as research objects to prepare steel slag-based biomass-activated carbon. The property of chlorine absorption was tested by a P-C-T adsorption device, and the inductively coupled plasma mass spectrometer (ICP-MS), X-ray fluorescence spectrometer (XRF), laser particle size meter (LPSA) and field emission scanning electron microscope (SEM) was used to test the concentration of the leached heavy metals, chemical composition, particle size distribution and microscopic morphology, respectively. Moreover, the mechanism of preparing steel slag-based biomass-activated carbon from special steel slag-discard walnut shells was elaborated from the microscopic level. The results show that the special steel slag ultrafine powder solution contains heavy metals such as Cd, Cu, Pb, Zn, Ni, Cr, As, etc., and the leaching toxicity content of Pb, Ni and Cr is higher than the leaching toxicity limit in the “Leaching Toxicity Identification Standard” (GB 5085.3—2007). Phosphoric acid has destructive structural characteristics, and anhydrous ethanol promotes dispersion, which is conducive to eliminating the gravitational force between the micronized particles and improving the dispersion of the discard walnut shell ultrafine powder and the special steel slag ultrafine powder. The magnetic Fe2O3 contained in the special steel slag ultra-fine powder and the catalytic CuO and MnO form a synergistic effect, which is conducive to the formation and enrichment of chlorine gas on the surface of the steel slag-based biomass-activated carbon, and improves the adsorption capacity of chlorine gas. The adsorption capacity of steel slag-based biomass-activated carbon to chlorine gas shows a tendency to decrease slightly and then decrease greatly with the rise of the ambient adsorption temperature. The excessively high adsorption ambient temperature will enhance the activity of chlorine molecules, resulting in the analytical phenomenon of chlorine adsorbed by steel slag-based biomass activated carbon. The activated carbon formed during the activation treatment and roasting process of discarding walnut shell ultrafine powder not only wraps the special steel slag ultrafine powder but also solidifies the heavy metals in the special steel slag ultrafine powder.