Alkali metals were found to be poisonous to the Ce-Ti oxide catalyst. The NO conversion of the catalyst decreased by more than 80 % at temperatures from 250 to 450 °C with the doping of either 3.71 atom % K or 3.74 atom % Na. A combined experimental and theoretical study was used to reveal the mechanism of the deactivation caused be the alkali metals. The results indicated that the doping of alkali metal atoms on the catalyst surface greatly decreases the surface acidity and reducibility. XPS characterization revealed that the interaction between the alkali atoms and the oxygen atom of cerium oxide inhibits the transformation between Ce3+ and Ce4+, which effects a decrease in the proportion of Ce3+. The theoretical results show that the alkali atoms strongly interact with the cerium oxygen and titanium oxygen, which causes the degradation of reducibility and surface acidity, respectively. This causes a serious deactivation of the Ce-Ti oxide catalyst.