Abstract: The most commonly used method for industrial flue gas denitrification is selective catalytic reduction (SCR). However, the catalyst preparation is complex and expensive. The iron and steel industry produces large amounts of waste containing metal oxides that can be used as active catalytic components for SCR of nitrogen oxides. In this study, a novel catalyst for SCR of nitrogen oxides was prepared by roasting, sulfuric acid, and sulfuric acid-roasting modification of steelmaking sludge, which is used as the raw material. The physical and chemical properties of the catalysts from steelmaking sludge before and after modification were analyzed using Brunauer-Emmett-Teller analysis, scanning electron microscopy, X-ray diffraction, X-ray fluorescence, and temperature-programmed desorption of ammonia. It has been revealed that Fe, Mn, V, and Ti are the main active groups of the catalyst. Calcination can transform Fe₃O₄ to α-Fe₂O₃ with better denitrification activity, thus improving the catalyst reactivity. A high calcination temperature can cause a collapse of the pore structure of the catalyst, thereby decreasing the surface area and active sites and ultimately reducing the catalytic activity. The catalyst modified at the optimum calcination temperature of 400 °C has the highest catalytic activity at 350 °C and a denitrification efficiency of 57.6%. The sulfuric acid-modified catalyst has excellent catalytic activity. Sulfuric acid impregnation changes the surface morphology of the catalyst, reduces the grain size, generates numerous sulfate species, provides more acidic sites on the catalyst surface, and promotes catalyst performance. The 9 mol·L⁻¹ sulfuric acid-modified catalyst has the highest denitrification efficiency at 300 °C. Compared with the unmodified catalyst, the denitrification efficiency significantly increased from 22.9% to 88.5%. Conversely, a denitrification efficiency of 72.9% is measured for the catalyst modified by sulfuric acid and roasting modification, which is lower than that of the sulfuric acid-modified catalyst at 300 °C. This may be explained by the fact that sulfuric acid and roasting modification causes not only structural changes in the catalyst but also the decomposition of the generated sulfate species, thereby leading to catalytic efficiency reduction. This work shows a feasible preparation of a low-cost SCR catalyst for denitrification by roasting and acid modification using steelmaking sludge as the raw material, provides a theoretical basis for developing low-cost denitrification catalysts using metallurgical solid wastes and promotes clean production in the metallurgical industry.