Deficiency of appropriate host materials with high gravimetric density for hydrogen storage impedes the development of hydrogen economy and its downstream application, i.e., hydrogen fuel battery. Within the framework of density functional theory, we investigate systematically the performance of Li decorated Be3C2 for hydrogen adsorption and reversible storage. We find that Li atoms forms strong bond with the light-weight Be3C2 substrate without the issue of agglomeration, where dispersive decoration of Li on Be3C2 is calculated to be more energetically than Li metallic clusters. As for Li doped Be3C2 system, a maximum gravimetric hydrogen density of 10.79 wt% could be reached. Such high-capacity benefits from the light-weight of the host and its strong binding to H-2 molecules, in which both polarization and electronic hybridization mechanism play a significant role. Hydrogen storage under different pressure and temperature has also been discussed based on thermodynamic analysis and a practical capacity of 9.27 wt% could be expected under more realistic operation of hydrogen fuel battery (stored at 30 atm/25 degrees C and released at 3 atm/100 degrees C). Favorable hydrogen adsorption capability and desired storage/release dynamic performance endows Li decorated Be3C2 with promising applications in hydrogen energy storage field.