Arsenic, a toxic component in coal-fired flue gas, is poisonous to the commercial selective catalytic reduction (SCR) denitrification catalysts. To unveil the arsenic poisoning mechanism on commercial SCR catalysts, fresh and 1 year used arsenic-poisoned plate-type V2O5-MoO3/TiO2 catalysts from a coal-fired power plant in the Inner Mongolia Province of China were systematically analyzed with SCR activity and characterization experiments. The results indicated that the plate-type V2O5-MoO3/TiO2 catalysts possessed a certain ability to resist arsenic poisoning. The average denitrification efficiency of the poisoned catalysts was maintained over 70% at 350 degrees C, even though the arsenic content was as high as 7 wt %, compared to the denitrification efficiency of 87.35% for the fresh catalyst. Characterization results indicated that both physical and chemical factors resulted in the deactivation of catalysts by arsenic. The surface area and amount of surface acid sites of the used catalysts decreased, which inhibited the adsorption of ammonia. The redox capacity of the used catalysts also decreased as a result of the increase of tetravalent vanadium (V4+) and the decrease of surface chemisorbed oxygen. Furthermore, catalysts at different installation positions in the SCR system had different denitrification activities and deactivation mechanisms. The major deactivation factor for the catalysts in the top layer was physical blockage, while the chemical deactivation was dominant for the catalysts in the middle layer.