A modification method that combines thermal and oxidation treatments was used to improve catalytic activity of activated carbon (AC) which catalyzed ozonation of the aquatic contaminant humic acid (HA). As a consequence of modifying virgin AC, modified AC (ACN2O2N2) had good catalytic performance for ozonation of HA. Apparent first-order rate constants (kapp) were ACN2O2N2 (7.88 × 10−3 s−1) > virgin AC (3.28 × 10−3 s−1). This change was discussed in terms of three factors: textural property, graphitization degree, and surface chemical property. From analysis results, it was deduced that the surface chemical property (the concentration of surface groups) was the main factor that influenced catalytic activity. An increase in the concentration of hydroxyl groups on AC enhanced catalytic activity of AC in ozonation of HA. Effects of phosphate (both a ligand and a strong Lewis base) further confirmed that Lewis acid sites (hydroxyl groups) were the active centers for free radical reaction in catalytic ozonation of AC.