To study the effect of different modification reagents on activated carbon (AC), HNO3, H2SO4, H2O2, H2C2O4, and H3PO4 were chosen to modify AC. Then a series of Mn/AC catalysts were prepared by impregnation method. The denitrification performance and SO2 adsorption activities of these catalysts were tested. The results indicated that the catalyst prepared by HNO3-modified AC has both the highest NO conversion and SO2 adsorption activity. The NO conversion can reach 80% at 180 °C and SO2 saturated adsorption time can reach 30 min. To investigate the enhanced modification mechanisms of HNO3, the various modified samples were characterized by SEM-EDS, XPS, FTIR, and NH3-TPD. And the results show HNO3 can effectively increase the amount of chemical adsorbed oxygen on the surface of AC. And on the other hand, the nitrogen-containing functional groups introduced by HNO3 can promote the adsorption of NO, thus improving the denitrification performance. As a result, HNO3 can play a better role in activating AC, and the catalyst modified by HNO3 is more conducive to be used as the carrier of SCR catalysts.