In this work and for the first time, an alkali metal potassium (K) as a promoter was loaded on an activated carbon (AC) to be used for selective catalytic reduction of NO from industrial flue gas. The denitrification activity of K-modified AC samples was investigated and compared with those of acid- and Cu-modified AC samples. The three modifications can enhance the denitrification activity of AC samples. To investigate the enhanced mechanisms of denitrification and SO2 resistance, various AC samples were characterized by SEM, EDX, XRF, ICP, XRD, N-2 physisorption, H-2-TPR, XPS, in situ DRIFTS and TPD. For the acid modification, the content of phenolic hydroxyl increases and then promotes the adsorption of NH3. For the loading of Cu, the appearance of Cu2+/Cu+ sites with high catalytic activity promotes NH3 and NO adsorption and changes the reaction pathway. For the loading of K, the adsorption capacity for NO increases nearly 12 times higher than that on Raw-AC, of which 85.0% is chemical adsorption. Moreover, three kinds of active nitrate species additionally forms, including nitro compounds, monodentate nitrate and nitrous oxide, which promote the denitrification reaction path. AC-Cu has poor sulfur resistance due to the sulfation of Cu, while AC-K show good sulfur resistance due to the enhanced adsorption of NO. The catalytic dissociation activity of N2O on AC-K makes the formation and decomposition of N2O into a dynamic equilibrium. As a result, AC-K has a high N-2 selectivity, while AC-Cu has poor N2 selectivity.