A cluster/semiconductor model is built for exploring the role of noble metal clusters in a photocatalytic system. The incorporation of an atomically precise nanocluster, e.g. Ag44(SR)30, onto a large band gap semiconductor such as TiO2 allows to obtain a clear interface and thus simplify the system. The composite is employed for photocatalytic H2 generation. It’s found that changing the light source from visible light to simulated sunlight leads to an enhancement by three orders of magnitude. The H2 production rate reaches 7.4 mmol/h/gcatalyst which is five times higher than that of Ag nanoparticles modified TiO2 and even comparable to that of the similar conditioned Pt nanoparticle modified TiO2. Energy band alignment and transient absorption spectroscopy, together with other studies, reveal that the role of the metal clusters is different from both organometallic complexes and plasmonic-nanoparticles. A type-II heterojunction charge transfer route is achieved under UV-vis irradiation, in which the cluster serves as small band gap semiconductor. The type-II photosystem has a more efficient charge separation ability, which contributes significantly to the enhanced catalytic performance. This finding endows the clusters a broad platform as cocatalysts rather than merely photosensitizers in the applications of light energy conversion.