Abstract: Accurate determination of the state of hydrogen (SOH) in solid-state hydrogen storage materials is essential not only for optimizing hydrogen release kinetics and enhancing storage efficiency but also for ensuring system safety in practical applications. While most existing studies have concentrated on thermodynamics and kinetics, direct monitoring of residual hydrogen content, a parameter of critical engineering relevance, has rarely been reported. This highlights the urgent need to realize online SOH detection through new physical properties. In this study, we propose a non-invasive, real-time SOH monitoring strategy for magnesium hydride (MgH₂), based on optical properties and combining density functional theory (DFT)-based optical calculations with experimental validation. Using DFT, the optical properties of MgH₂ and its dehydrogenated form (Mg) were systematically calculated across the infrared, visible, and ultraviolet spectral ranges. Theoretical results revealed strong linear correlations between SOH and specific optical parameters, such as reflectance at 1200 nm and 550 nm and refractive index at 250 nm, with r² values exceeding 0.99 and mean absolute errors below 0.05. To validate these predictions, reflectance measurements were conducted at 940 nm, a wavelength identified as highly sensitive to hydrogenation, and a consistent decrease in reflectance with increasing hydrogen uptake was observed. The underlying mechanism was attributed to band structure evolution and electron density redistribution, supported by density of states analysis and Drude model interpretations. This work establishes a robust theoretical and experimental framework for optical SOH diagnostics, emphasizes the importance of residual hydrogen detection for advancing solid-state hydrogen storage from fundamental research toward practical engineering applications, and provides new insights into the design of intelligent, optically responsive hydrogen storage systems, paving the way for the development of spectroscopic SOH sensors in next-generation hydrogen energy technologies.