Hydrogen is widely regarded as an ideal alternative energy source because of its high efficiency, abundance, pollution-free and renewable properties. One of the main challenges is to find efficient materials that can store hydrogen safely with fast kinetics, favorable thermodynamics, and high hydrogen density under ambient conditions. The nanomaterial is one of the most promising hydrogen storage materials because of its high surface-to-volume rate, unique electronic structure and novel chemical and physical properties. In this study, the hydrogen storage properties of Na-decorated B_n (n = 3–10) clusters are investigated using dispersion-corrected density functional theory and atomic density matrix propagation (ADMP) simulations. The results show that Na atoms can stably bind to B_n clusters, forming B_nNa₂ complexes. The average binding energies (1.876–2.967 eV) of Na atoms on the host clusters are significantly higher than the cohesive energy of bulk Na (1.113 eV), which effectively prevents Na atoms from gathering on the cluster surface. Furthermore, when Na atoms bind to B_n (n = 3–10) clusters, electrons transfer from Na to B atoms, resulting in positively charged Na atoms. Hydrogen molecules are moderately polarized under the electric field and adsorbed around Na atoms through electrostatic interactions. The H–H bonds are slightly stretched but not broken. The Na-decorated B_n clusters can adsorb up to 10 hydrogen molecules with average adsorption energies of 0.063–0.095 eV/H₂ and maximum hydrogen storage densities reaching 11.57%–20.45%. Almost no structural change is observed in the host clusters after adsorbing hydrogen. Molecular dynamics simulations reveal that the desorption rate of hydrogen molecules increases with temperature rising. At ambient temperature (300 K), B_nNa₂ (n = 3–8) clusters achieve complete dehydrogenation within 262 fs, while B₉Na₂ and B₁₀Na₂ clusters exhibit a dehydrogenation rate of 90% within 1000 fs. The Na-decorated B_n (n = 3–10) clusters not only exhibit excellent properties for hydrogen storage but also enable efficient dehydrogenation at ambient temperature. Thus, B_nNa₂ (n = 3–10) clusters can be regarded as highly promising candidates for hydrogen storage.