Abstract：Density functional theory (DFT) are used to carry out the trends in the adsorption chemistry of NOx (x＝1, 2), CO2, H2O on the BaO(001) surface. NO adsorbs on the surface O atom in the formation of species|CO2 is adsorbed on the surface O via its C atom and a carbonate-type structure is identified|while for the H2O adsorption, it is observed that H2O adsorbs dissociatively, forming hydroxyl on the surface|as for the adsorption structures of NO2, two important geometries are identified: one is N-bonded adsorbate, and the other is O-bonded adsorbate. The adsorption strength increases in the order NO≈H2O＜CO2＜NO2. For all adsorption species, the adsorption energies are markedly descending as the surface coverage increasing. It is worth noting that the simultaneous formation of surface / pairs by adsorptions of two NO2 is most energetically favorable, and the cooperation effect probably could be the driving force for the BaCO3 and Ba(OH)2 eventual conversion into Ba(NO3)2.