In this work, to develop better OCM catalysts, especially with improved ethylene selectivity, the promotional effects of BaBr2 on defect BaSnO3 has been investigated by changing the BaBr2/BaSnO3 molar ratio. XRD and Raman have substantiated that BaBr2 and BaSnO3 co-exists in all the BaBr2-modified catalysts. Compared with pure defect BaSnO3, all the modified catalysts exhibit significantly improved OCM performance, particularly with extremely high ethylene selectivity. Electrical conductivity measurement have proven that defect BaSnO3 is a p-type semi-conductor favouring OCM reaction, whose surface possesses abundant vacancies. O-2-TPD, CO2-TPD and electrical conductivity dependence on O-2 partial pressure results have indicated that BaBr2 addition can enhance the surface oxygen vacancy concentration of BaSnO3, which facilitates the conversion of surface lattice O-2(-) into OCM reactive oxygen sites, such as O-2(2-) and O-2(-) anions. At the same time, a larger amount of moderate strength basic sites have been formed. EPR, in situ Raman and XPS results have demonstrated that the quantities of these active oxygen sites can be evidently improved by BaBr2 addition. More importantly, a type of new O-2 delta- (0 < delta < 1) sites beneficial to OCM reaction have also been formed. Notably, XPS results have revealed that the amount of O-2(2-) sites, which can directly convert CH4 into ethylene through carbene intermediates, has been remarkably increased due to the interaction between BaBr2 and BaSnO3. As a consequence, the OCM reaction performance, especially ethylene selectivity and yield on the BaBr2-modified BaSnO3, has been significantly improved.