Efficient hydrogen production via photocatalysis with high utilization efficiency of Pt cocatalyst is of great importance for sustainable development. In this work, we report an in situ auto-reduction strategy to encapsulate highly dispersed Pt clusters inside the cages of MIL-125-NH₂. The amino groups in MIL-125-NH₂ first react with formaldehyde to form reducing groups (i.e., –NH-CH₂OH), which can in situ auto-reduce the confined Pt²⁺ ions to ultrasmall Pt clusters within the cavities. With optimized Pt content, photocatalytic H₂ production over the obtained Pt(1.5)/MIL-125-NH-CH₂OH catalyst with 1.43 wt.% Pt loading achieved as high as 4, 496.4 µmol·g⁻¹·h⁻¹ under visible light (λ > 420 nm) due to the facilitated transfer and separation of the photo-induced charger carriers arising from the synergetic effects between highly dispersed Pt clusters and MIL-125-NH-CH₂OH framework. This in situ auto-reduction strategy may be extended to encapsulate various kinds of metal or alloy clusters/nanoparticles within amino-functioned metal-organic frameworks (MOFs) with superior properties and excellent performance.