Abstract: With steel slag and biomass waste material as the research object, biomass waste material was modified by metal oxide in steel slag to obtain ecological activated carbon. The influences of steel slag type, grinding time of steel slag, and the amount of steel slag ultrafine powder on the formaldehyde degradation performance of ecological activated carbon were studied. The chemical composition of steel slag, mineral composition of steel slag, particle size distribution of steel slag, structural composition of steel slag ultrafine powder, the pore structure of ecological activated carbon, and the microstructure of ecological activated carbon were characterized by X-ray fluorescence X-ray diffraction, laser particle size distribution analysis, Fourier-transform infrared spectroscopy, Brunauer-Emmett-Teller analysis, and scanning electron microscopy, respectively. The results show that the prepared ecological activated carbon show good formaldehyde degradation performance and reasonable economy; the degradation rate of formaldehyde after 10 h is 57.5%; when steel slag is electric furnace slag, the grinding time of the steel slag is 90 min, and the amount of steel slag ultrafine powder is 20 g. High contents of Fe and Mn were present in the electric furnace slag. Iron promoted the concentration of a large amount of formaldehyde in the porous structure of activated carbon, and Mn catalyzes the degradation of enriched formaldehyde, realizing the synergistic effect of adsorption degradation and catalytic degradation. Appropriately extending the grinding time of the steel slag can significantly reduce the particle size of the steel slag ultrafine powder and improve the particle size distribution uniformity of the steel slag ultrafine powder, which is beneficial to increasing the degradation area of steel slag ultrafine powder, activated carbon, and formaldehyde. An appropriate amount of steel slag ultrafine powder can improve the pulverization rate of ecological activated carbon and offset the decline of activated carbon adsorption performance due to the decrease of porosity and specific surface area of the activated carbon.