Activated carbon with developed porous structure and abundant specific surface area was an efficient technical means to remove formaldehyde from indoor air, but it exists the problems of high production cost, unfavorable to the sustainable development of the ecological environment, short service life and easy to cause secondary pollution of indoor environment after failure. Steel slag tailings are the main solid waste in metallurgical industry, with the production of 15%~20% of crude steel. The utilization ratio is quite low and only reaches 10% of steel slag tailings production due to limited technology. Meanwhile, steel slag tailings are disposed in direct stacking and landfill in general since the management system is not perfect, which pollutes land source, underground water source and air quality. In the face of the above problems, the development of low price and superior performance of modified activated carbon has become not only one of the main methods to achieve the high value-added utilization of metallurgical solid waste and the sustainable development of resources, but also one of the main methods to achieve the greatly reduce the production cost of modified activated carbon and improve economic benefits. In this paper, with superfine powder of hot braised slag as the research object, chemical composition of hot braised slag and mineral composition of hot braised slag were characterized by X-ray fluorescence spectrometer (XRF) and X-ray diffractometer (XRD), respectively. According to the main chemical composition and main mineral composition of hot braised slag, hot braised slag chemical composition modified activated carbon and hot braised slag mineral composition modified activated carbon was prepared. Performance of hot braised slag chemical composition modified activated carbon and hot braised slag minerals composition modified activated carbon were tested by referring to Indoor decorating and refurbishing materials-Limit of formaldehyde emission of wood-based panels and finishing products (GB 18580—2017), in order to the influences of main chemical composition and main mineral composition on formaldehyde degradation performance of modified activated carbon. The results show that the main chemical composition of hot braised slag are CaO, Fe2O3, SiO2, P2O5, MgO, MnO and Al2O3, among Fe2O3 and MnO modified activated carbon can improve the formaldehyde degradation performance. The main mineral composition of hot braised slag are Fe3O4, 3CaO·SiO2, 2CaO·SiO2 and RO phase, among Fe3O4 and RO phase modified activated carbon can improve the formaldehyde degradation performance. The element Fe exists as the chemical composition of Fe2O3 and mineral composition of Fe3O4, RO phase in hot braised slag, the element Mn exists as the chemical composition of MnO and mineral composition of RO phase in hot braised slag, the synergistic effect of Fe element and Mn element can improve the formaldehyde degradation performance of hot braised slag modified activated carbon. Hot braised slag modified activated carbon not only realized the high value-added utilization of metallurgical solid waste, but also put forward the new indoor air formaldehyde management technique of “treating danger by waste”.