The deactivated denitration catalysts used in a coal-fired power plant were washed and then immersed in activation solutions with oxalic acid and ethanolamine respectively as cosolvents for investigating the regeneration effect. The fresh, deactivated, and two regenerated catalysts were analyzed with XRF, N2-physisorption, in-situ pyridine adsorption, NH3-TPD, Raman, XPS, H2-TPR, and then were tested in a fixed bed denitration catalytic reactor. The major deactivation of denitration catalysts was because that the K, Na from fly ash caused loss of specific surface area, pore volume, Lewis acid sites, active VO x species, and ratio of V5+, and weakened the catalysts’ oxidation-reduction ability. Meanwhile, the two regenerated catalysts with the same vanadium content showed great difference on the denitration efficiencies. Ethanol-cat, regenerated by ethanolamine-assisted activation solution had denitration efficiency over 97% of that of the fresh catalyst. However, Oxalic-cat, regenerated by oxalic acid-assisted activation solution showed almost no regeneration effect. This was due to the vanadium species with different states in the two activation solutions. The vanadium species in ethanolamine-assisted activation solution could recover Lewis acid sites, ratio of V5+ and active VO x species of the deactivated catalysts, and enhance the oxidation-reduction ability of catalysts effectively, but the vanadium species in oxalic acid-assisted activation solution could hardly attain these results.