Abstract: To address the issues of insufficient adsorption sites and poor stability in metal-organic frameworks (MOFs) for hydrogen storage, a series of Pt-Mg-MOF-74-R composite materials with varying Pt loadings and defect degrees were synthesized using an in-situ platinum (Pt) incorporation strategy combined with thermal reduction, based on the Mg-MOF-74 framework. The micromorphology and crystal structure of the materials were systematically characterized by XRD, FT-IR, SEM, TG, TEM, and XPS. The regulatory effects of thermal reduction temperature and Pt loading on the framework defect formation and metal valence states were investigated. The results indicate that the thermal reduction process effectively reduces Pt ions into highly active Pt0 nanoparticles while removing solvent molecules and inducing partial ligand loss, thereby constructing abundant MgO5 open metal sites (OMSs). Hydrogen storage performance tests show that the 4-Pt-Mg-MOF-74-R200 sample achieves a gravimetric hydrogen storage capacity of 0.35 wt% with an adsorption enthalpy of -22.35 kJ/mol at 298 K and 100 bar. Mechanism analysis confirms that the “hydrogen spillover” effect triggered by Pt0 promotes the dissociation of \textH_2 and accelerates the migration of hydrogen atoms to adjacent MgO5 sites. This synergistic dissociation-adsorption mechanism is the key to enhancing the hydrogen storage performance.