To prepare supported catalysts with high loading and high dispersion of active metals is a crucial challenge in heterogeneous catalysis. Here we report the fabrication of very reactive catalyst that contains finely dispersed copper with 20 wt% content supported on high-surface area CeO₂ for the water–gas shift (WGS) reaction. This copper-ceria catalyst shows superb activity with a CO consumption rate of 116.7 μmol_CO g_cat^-1 s⁻¹ at 300 ℃, nearly one order of magnitude more active than other copper-based catalysts ever reported. Based on the precise and integrated characterizations, the oxidized not the metallic Cu are proved to be active centers where the adsorbed CO directly reacted with bridged —OH groups to finish the WGS reaction. The oxidized Cu sites are confirmed to be originated from the activation effect from Cu to CeO₂, which promotes the oxygen of CeO₂ diffuse from the bulk to surface, making Cu oxidized even under reductive atmosphere.