Barium promoter is widely used in the secondary ammonia synthesis catalysis, which could greatly improve the performance of a catalyst. Although barium oxide is confirmed as the main component of barium promoter, the existence of metallic barium has been argued. In order to theoretically clarify this issue, the first principles calculations have been performed to study the geometric stability and the nitrogen adsorption properties of small BaxOy cluster-modified Ru(0001) surface. It is found that Ba2O cluster is more stable than other small clusters or atoms (BaO2, BaO, Ba and O) on the Ru(0001) surface under the condition that the pressure rate of H2O/H2 is below 1‰. This implies that BaO promoter could be partially reduced by hydrogen gas in the experiment. According to the results of the projected density of states and charge difference induced by modification of cluster, the O atom in Ba2O cluster gains electrons from dz2 orbit of the underlying Ru atom, and forms O–Ru bonds; while Ba atom in Ba2O clusters transfers electrons to the nearest Ru atoms and forms Ba-Ru metallic bonds. As the adsorption of nitrogen is an initial reactant in ammonia synthesis, we also study the nitrogen adsorption properties near the Ba2O cluster. Compared with the different chemical properties of O and Ba atoms, the adsorption properties of nitrogen molecules on the sites close to O and Ba atoms are similar. The nitrogen adsorption energies at the corresponding sites are calculated to be 0.88 and 0.78 eV, respectively. The bond lengths of nitrogen molecules are about 0.187 nm near O atom, and 0.190 nm near Ba atom, both of which are shorter than those on a clean surface (～ 0.197 nm). And the stretching vibrational frequency of a nitrogen molecule is calculated to be 1888 cm-1 near the O atom, 1985 cm-1 near the Ba atom, both of which are also less than those on a clean surface (～ 2193 cm-1). This suggests that Ba2O cluster may weaken the bond strength of nitrogen molecules. According to the charge difference induced by nitrogen adsorption, the electrostatic interactions of Ba2O clusters increase the occupation of π antibonding orbital and the electric polarization of the nitrogen molecule, and thus weaken the N–N bond.