Photocatalytic hydrogen peroxide (H₂O₂) production is a sustainable alternative to the conventional industrial processes but it is often limited by inefficient light harvesting and sluggish interfacial charge transfer. Covalent organic frameworks (COFs), together with ultrasmall metal nanoclusters (MNCs) that provide abundant and well-defined active sites, have emerged as promising candidates for advanced photocatalytic systems. However, constructing MNCs@COF composites with high metal loading while elucidating the underlying structure and property relationship remains a significant challenge. Herein, a novel AuNCs-embedded COF composite (AuNCs@NHC-COF) was developed by employing N-heterocyclic carbene (NHC)-coordinated Au(I) complexes as precursors and incorporated in an imine-linked COF scaffold via an in-situ reduction strategy. Uniform AuNCs with an ultrasmall size of 1.66 ± 0.09 nm were densely confined inside the COF pores, achieving the high loading content of 22.67 wt%. The incorporation of Au nanoclusters significantly enhanced visible-light absorption, charge-carrier separation, and interfacial charge-transfer kinetics compared with the parent COF materials. By precisely tuning the photocatalytic conditions, AuNCs@NHC-COF exhibited an outstanding H₂O₂ production rate of 16565.55 μmol g^-1 h^-1 in a benzyl alcohol/water biphasic system. A two-electron oxygen reduction reaction mechanism (2e⁻ ORR) was proposed based on experimental investigations and density function calculations. Moreover, a systematic correlation between metal active sites and photocatalytic H₂O₂ generation activity was clarified. This work provides an effective design strategy of active MNCs@COF composites for artificial H₂O₂ photogeneration.