Abstract: Metal-organic frameworks (MOFs) hold great promise for hydrogen storage owing to their high specific surface areas, tunable pore structures, and chemical modifiability. Mg–MOF–74 has attracted considerable attention because of its unique structural features: Mg ions coordinate with five oxygen atoms and one solvent molecule, and open metal sites (OMSs) are generated upon activation to remove the solvent. This material possesses a highly polarized pore environment, offering inherent potential for hydrogen storage. However, existing studies have shown that the hydrogen uptake of Mg–MOF–74 remains below 0.15% weight present at 298 K and 7 MPa, and its practical application is severely limited by sluggish H₂ diffusion kinetics. Introducing noble metal components can trigger the hydrogen spillover effect, in which H₂ molecules dissociate into H atoms and migrate to the support, significantly enhancing the storage capacity. To address the insufficient adsorption sites and stability issues of MOF-based hydrogen storage materials, a series of Pt–Mg–MOF–74–R composites with varying Pt loadings and defect levels were fabricated via in situ Pt introduction using K₂PtCl₄ as the Pt source, followed by thermal reduction in an Ar/H₂ atmosphere with Mg–MOF–74 as the host. Systematic characterization by XRD, FT–IR spectroscopy, SEM, TGA, TEM, and XPS revealed the effects of the reduction temperature and Pt loading on the framework defect formation and metal valence states. The thermal reduction process effectively reduced the Pt ions to highly active Pt⁰ nanoparticles while removing solvent molecules and inducing partial ligand loss, leading to the construction of abundant MgO₅ open metal sites. The FT–IR results showed that the Pt-loaded materials exhibited peak positions similar to those of pristine Mg–MOF–74, with no new functional groups detected, indicating that Pt was predominantly present in a physically loaded state within the n–Pt–Mg–MOF–74 framework. Hydrogen storage tests showed that 4–Pt–Mg–MOF–74–R₂₀₀ achieved a gravimetric uptake of 0.35% weight present at 298 K and 10 MPa, which was 2.5 times that of pristine Mg–MOF–74 under identical conditions. The adsorption enthalpy reached −22.35 kJ·mol^–1, 3.8 times higher than that of the original Mg–MOF–74, with the thermal reduction process playing a key role. Mechanistic analyses indicated that thermal reduction increased the content of well-dispersed Pt⁰, facilitating hydrogen spillover by dissociating H₂ molecules into H atoms. Simultaneously, thermal reduction removed linkers without collapsing the framework, increasing the number of MgO₅ open metal sites available for hydrogen binding and optimizing the mass transfer pathways and coordination environment, thereby providing more transport routes and adsorption sites for the H atoms generated via spillover. The two mechanisms reinforce one another and collectively enhance the room-temperature hydrogen storage capacity of 4–Pt–Mg–MOF–74–R₂₀₀. Notably, increasing the reduction temperature from 200 °C to 300 °C caused partial pore collapse, a decrease in the specific surface area, and a significant increase in the mesoporosity. These structural changes adversely affected the hydrogen storage performance, with 4–Pt–Mg–MOF–74–R₂₀₀ clearly outperforming 4–Pt–Mg–MOF–74–R₃₀₀. Although thermal reduction generates highly active defect sites that substantially enhance the hydrogen adsorption capacity, excessive temperatures compromise the pore architecture. The balance between defect engineering and structural integrity indicates that an optimal reduction temperature of 200 °C maximizes the hydrogen storage performance by preserving an adequate pore structure and surface area while optimizing the quantity and distribution of defect sites. This study primarily reveals the synergistic mechanism of "hydrogen spillover + defect sites" in MOF-based hydrogen storage.