Based on van der Waals corrected density functional theory, we show that Na atoms acting as decoration metals are not inclined to form clusters due to a large binding energy of 3.31 eV, indicating a promising good reversible hydrogen storage. Both the polarization mechanism and the orbital hybridizations contribute to the adsorption of hydrogen molecules (storage capacity of 4.4 wt%) with optimal adsorption energy of 0.25 eV/H2. Additionally, the dimerization of these isolated B36 does not remarkably affect the number of adsorbed H2 per Na atom. Our results may serve as a guide in the design of new hydrogen storage materials based on low-dimension boron clusters.