The photocatalysis process follows two fundamental sequences: reactant adsorption and surface photocatalysis. In this work, Bi metal-deposited Bi2GeO5 (Bi@BiGeO) is prepared as a model photocatalyst to understand the adsorption and activation mechanisms of the reactants and the photocatalytic oxidation of NO under visible light irradiation. Density functional theory and analytical approaches are employed to reveal the electronic structure and photo-induced carrier transfer processes. The introduction of Bi metal and the generation of oxygen vacancies (OVs) in BiGeO were achieved simultaneously via a facile method. The Bi metal served as a visible light antenna and as an electron sink and promoted the carrier separation and transfer. The OVs promote reactant (H2O and O-2) activation, thereby reinforcing the generation of reactive oxygen species (ROS). The NO molecules are actively adsorbed at the coordinative unsaturated sites on Bi@BiGeO and get activated via electron exchange. The photocatalytic NO oxidation mechanism on Bi@BiGeO is revealed based on the reaction intermediates and final products monitored using in-situ FTIR. This work highlights the importance of reactant activation as a new strategy for the design of highly efficient photocatalysts to overcome the bottlenecks in environmental applications.