CaO decreased the performance of the urea-selective non-catalytic reduction deNO(x) process by affecting the conversion of urea pyrolysis products. The effect of CaO on HNCO conversion was studied in a pyrolysis/gas-solid reaction system along with diffuse reflectance infrared Fourier transform spectroscopy and scanning electron microscopy. CaO was found to react with HNCO, and HNCO conversion and product selectivity changed with the temperature and O-2. Mechanism analysis showed that CaO first reacted with HNCO to produce Ca(NCO)(2). Ca(NCO)(2) decomposed to CaCN2 and CO, at high temperatures. CaCN2 stably existed because of its high thermal stability. In the presence of O-2, Ca(NCO)(2) decomposed and was also oxidized to CaO, N2O, and CO2. In addition, produced CaCN2 was oxidized to CaO, NO, and CO. CaO was reproduced in Ca(NCO)(2) and CaCN2 oxidation; thus, the reaction between CaO and HNCO was accelerated and HNCO conversion was increased. The selectivity of products was determined by Ca(NCO)(2) decomposition, Ca(NCO)(2) oxidation, and CaCN2 oxidation.