We demonstrate an alkali modification process to produce highly dispersed ultrafine Pt nanoclusters with metallic Pt$0$ and oxidized Pt$\mathrm{<mrow\; data-mjx-texclass="ORD">2+</mrow>}$ species as co-catalyst anchored on nanosheet-constructed yolk-shell TiO$2$ (NYTiO$2$-Pt) acting as light harvesting reactor for highly efficient photocatalytic H$2$ production. Benefiting from the high surface area, highly dispersed ultrafine Pt nanoclusters (~0.6 nm) with Pt$0$ and Pt$\mathrm{<mrow\; data-mjx-texclass="ORD">2+</mrow>}$ species and special nanosheet-constructed yolk-shell structure, this novel light harvesting reactor exhibits excellent performance for photocatalytic H$2$ production. The NYTiO$2$-Pt-0.5 (0.188 wt% Pt) demonstrates an unprecedentedly high H$2$ evolution rate of 20.88 mmol h$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ g$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ with excellent photocatalytic stability, which is 87 times than that of NYTiO$2$-Pt-3.0 (0.24 mmol h$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ g$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, 1.88 wt% Pt), and also much higher than those of other TiO$2$ nanostructures with the same Pt content. Such H$2$ evolution rate is the highest reported for photocatalytic H$2$ production with such a low Pt content under simulated solar light. Our strategy here suggests that via alkali modifying the photocatalysts, we can not only enhance the H$2$ production for solar energy conversion but also significantly decrease the noble metal content for cost saving.