Developing efficient catalysts for the selective catalytic reduction of NOx by CO (CO-SCR) is the key challenge for commercializing this technology. Ag-based catalysts with relatively low costs are promising but widely believed to be not efficient enough for this reaction. Here, we demonstrate that atomically dispersed Ag supported on ordered mesoporous WO3 (m-WO3) can serve as a highly active catalyst for CO-SCR under O2-containing conditions. By altering the amount of the Ag precursor, the local environment of the Ag atom coordinated with the O atom can be tailored. Furthermore, at 250 °C and an O2/CO ratio of 2.5:1, 0.3Ag/m-WO3 (0.3 wt % Ag) with six-coordinated Ag-O exhibited much better catalytic performance than 5 Ag/m-WO3 (5 wt % Ag) with two-coordinated Ag-O (e.g., 0.43 vs 0.02 molNO gAg-1 h-1 in the reaction rate) and previously reported Ag-based catalysts in the literature. The theoretical calculations confirm that the six-coordinated Ag atoms in 0.3Ag/m-WO3 possess a more positive oxidation state and a higher d-band center than the two-coordinated Ag atoms in 5Ag/m-WO3, promoting its bonding strength with co-Adsorption of the critical intermediates of N2O∗ and CO*. This work provides a feasible route for regulating the local environment of a Ag single atomic catalyst to enhance its catalytic property for CO-SCR.