Abstract: The poisoning of a commercial selective catalytic reduction (SCR) catalysts by alkali (K) and alkaline earths (Ca) has been simulated in the laboratory. The techniques of N₂ adsorption, scanning electronic microscopy, X-ray photoelectron spectroscopy, NH₃-temperature program desorption, H₂-temperature program reduction were used to identify the changes of physical chemical characteristics of the catalysts before and after the simulated poisoning. The results indicated that the poisoning of K and Ca did not damage the basic pore structure of the SCR catalyst, but decreased the BET surface area and pore volume. The poisoning by K and Ca changed the chemical valence state of V and decreased the reducibility of V. The poisoning by K and Ca decreased the amount of chemically adsorbed oxygen on the catalyst surface as well as acidity of the catalysts. The poisoning by K and Ca lowered the SCR activity of the catalysts and the poisoning by Ca was more serious than K.