Activated carbon with developed porous structure and abundant specific surface area was prepared from wood, bamboo and other biological resources, which has the problems of high production cost, being unfavorable to the sustainable development of ecological environment, short service life and being easy to cause secondary pollution of indoor environment after failure. Metallurgical solid waste and biomass waste as a major by-product of industrial production and agricultural production, their use is difficult to achieve, with low added value and high cost, leading to the fact that a large number of metallurgical solid waste and biomass waste cannot be disposed in direct stacking and landfill, causing not only the pollution of ecological environment, but also a great waste of potential resources. In the face of the above problems, the development of low price and superior performance of ecological activated carbon with metallurgical solid waste and biomass waste has become not only one of main methods to achieve the high value-added utilization of metallurgical solid waste and biomass waste and the sustainable development of resources, but also one of main methods to achieve the great reduction of the production cost of modified activated carbon and improve economic benefits. In this paper, walnut shell and electric furnace slag were studied firstly, ecological activated carbon for formaldehyde degradation was prepared by metal oxides contained in the electric furnace slag modified to treat the biomass waste, and performance of ecological activated carbon was tested by referring to Indoor decorating and refurbishing materials-Limit of formaldehyde emission of wood-based panels and finishing products (GB18580—2017). Element content was characterized and analyzed by X-ray photoelectron spectroscopy (XPS), chemical component was characterized and analyzed by X-ray fluorescence spectrometer (XRF), composition structure was characterized and analyzed by fourier transform infrared spectrometer (FTIR), mineral composition was characterized and analyzed by X-ray diffractometer (XRD), microstructure was characterized and analyzed by scanning electron microscope (SEM), particle size distribution was characterized and analyzed by laser particle analyzer (LPSA) and pore structure was characterized and analyzed by specific surface area and pore size distribution analyzer (BET), so as to reveal the mechanism of preparing ecological activated carbon from walnut shell and electric furnace slag, and the degradation mechanism of formaldehyde by ecological activated carbon. The results showed that ecological activated carbon with good degradation performance of formaldehyde is prepared by walnut shell ultrafine powder and electric furnace slag ultrafine powder, not only realizing the high value-added utilization of metallurgical solid waste and biomass waste, but also putting forward the new indoor air formaldehyde management concept of “treating danger by waste”. Electric furnace slag ultrafine powder is well wrapped in the layered structure of ecological activated carbon, to promote the pulverization rate of ecological activated carbon and forming particles with small particle size can improve the degradation area of ecological activated carbon and formaldehyde. Fe element, Mn element and Ti element are contained in electric furnace slag ultrafine powder; the magnetic property of Fe element induces the formation and enrichment of large amounts of formaldehyde on the pore structure surface of biomass activated carbon, and Mn element and Ti element catalyze the degradation of the enriched formaldehyde, realizing the synergistic effect of adsorption degradation and catalytic degradation.