Abstract: A molecular modeling based on the density functional theory (DFT) and the transition state theory (TST) was performed to investigate the influence of biomass gas CO on the N₂O decomposition catalyzed by CaO during reburning in the circulating fluidized bed boiler. The model for N₂O adsorption onto the CaO(100) surfaces were constructed; and the processes of the N₂O decomposition on the CaO(100) surface and the surface recovery of CaO(100) were investigated. The results illustrate that the energy barrier of N₂O decomposition on the CaO(100) surface is much lower than that in homogeneous case, and CaO is therefore able to catalyze the N₂O decomposition. The atomic O from N₂O decomposition can poison the active sites O atom on the CaO(100) surface, while biomass gas CO can promote the regeneration of the active sites on the surface of CaO(100), which is beneficial for CaO to catalyze the N₂O removal.