Abstract: The reaction mechanism of CaO with HCN during low temperature sludge pyrolysis was investigated by density function theory. The geometric optimization and frequency calculations of reactants, products, intermediates and transition state in the reaction pathway were performed at B3LYP/6-311++(3df, 2p) level; single point energy calculation was performed at CCSD (T)/cc-pVQZ level and the total energy was corrected by zero-point energy at B3LYP/6-311++(3df, 2p) level. The results indicate that largest energy barrier (310.33 kJ/mol) appears in proton transition process after 2 HCN molecules are adsorbed on CaO. Arrhenius equation for each step was fitted by classical transition state theory and the reaction rate was calculated at 3 typical temperatures. The results suggest that proton transition is the rate-determining step; moreover, the promoting effect of CaO on HCN is enhanced with the increase of temperature.