Ag/CeO2 nanosphere catalysts were successfully tuned by sodium doping to promote low-temperature catalytic oxidation of formaldehyde (HCHO). It was observed that sodium-promoted Ag/CeO2 nanosphere catalysts exhibited relatively high intrinsic catalytic activity. Different characterization techniques were performed on the catalysts to elucidate the physicochemical properties that contributed to the improvement of the catalytic performance. Due to electron charge transfer from Ag to CeO2 nanospheres, Ag species should comprise of Ag2O as main components along with parts of metallic Ag over CeO2 nanospheres, while remaining as metallic Ag over conventional CeO2 bulk particles. The well-dispersed Ag species on CeO2 nanospheres could activate surface oxygen from CeO2, and facilitate the surface reduction of CeO2 nanospheres. The doped sodium might exist in the forms of NaHCO3 and Na2CO3 compounds, which could cause that surface hydroxyl (OH-) concentrations significantly increased on the surface of CeO2 nanospheres. The presence of sodium compounds could prevent hydrogen spillover from Ag to CeO2 supports, attesting to the inhibiting role of sodium species in the reduction process. However, with the introduction of sodium into Ag/CeO2 nanospheres, surface hydroxyl groups would become important reaction species and directly react with adsorbed surface formate species (HCOO-) to form H2O and CO2. This reaction route was inconsistent with that over Ag/CeO2 nanospheres, on which bidentate HCOO- were the key intermediates during catalytic oxidation of HCHO, and the further oxidation of HCOO- by activated surface lattice oxygen was the rate-determining step. These results provide guidance for designing highly efficient and economic silver-based catalysts for HCHO catalytic oxidation.