Two commercial catalysts at different temperatures were selected, with cement kiln ash physically loaded on them, to investigate the ash- and alkali-poisoning effect on selective catalytic reduction (SCR) catalysts. Ash loading on catalysts obviously decreased the amount of NO conversion. H2O vapor increased the ash-poisoning effect on catalysts, whereas SO2 addition apparently decreased the ash-poisoning effect because the NO conversion gradually returned to the same activity level as that for the catalysts without ash loading. In situ Diffuse Reflection Infrared Fourier Transform spectroscopy (DRIFT) was adopted to clarify this phenomenon. It was found that new Lewis acid sites and Bronsted acid sites had formed on the catalyst surface after SO2 treatment, and the new Bronsted acid sites enhanced the formation of NH4+. Moreover, the decomposition behavior of NH4HSO4 on the catalyst surface formed from HSO4- enhanced the NO conversion performance because of the reactions of NH4+ or NH3 from NH4HSO4 with gaseous NO.