Photocatalytic production of H₂O₂ in pure water holds great potential for sustainable industry. However, this prospect is greatly hindered by its sluggish reaction kinetics involving multistep electron transfer and unstable intermediates. Herein, the fully exposed silver (Ag) clusters were anchored on poly(heptazinimide) (PHI) catalyst, forming a new kind of fully exposed cluster catalysts (FECCs), which can boost photocatalytic generation of H₂O₂ in pure water, with a rate up to 1075.5 µmol g⁻¹ h⁻¹, surpassing its Ag single atoms counterpart and many recent-reported state-of-the-art photocatalysts. In situ characterization and DFT analysis showed that fully exposed Ag clusters can form the electron-rich centers and optimize the binding energy of the O₂ and proton H, which in turn enhances the protonation process of *OOH, reducing the energy barrier of key steps, and finally leading to a high yield of H₂O₂. Interestingly, its Ag single-atom counterpart was favorable for decomposing the H₂O₂ into ·OH. Thus, Ag FECCs achieve significantly enhanced tetracycline degradation efficiency with the strategic incorporation of Ag single atoms.