First principle calculations predicted that Li atoms can be uniformly adsorbed on both sides of BC3 sheet without clustering. After the coverage of adsorbed Li atoms approaches 33.3%, Li+BC3 complex attains a largest hydrogen storage ability of 12.57 wt.%. Thermodynamic analysis confirms that at room temperature (300 K) and pressure in the range of 115–250 atm, Li+BC3 complex can have the hydrogen storage capacity mentioned above. These values satisfy not only the DOE (U.S.) requirement but also the security needs in application.