Hydrogen is an excellent clean energy and the use of two-dimensional (2D) porous materials to increase the storage capacity of H2 molecules has become an effective research direction. In this work, a new 2D B4N4 monolayer had designed and constructed to explore the hydrogen storage performance using density functional theory (DFT) method. The calculations showed that the B4N4 monolayer had great thermodynamic and kinetic stability. The introduction of Li atoms on the B4N4 monolayer surface significantly improved the storage capacity of H2 molecules. The double-sided B4N4 monolayer decorated with 8Li atoms could take up 24H2 with average adsorption energy of −0.16∼-0.28 eV. The storage gravimetric density could reach up to 9.58 wt%, which satisfied the goal of 6.50 wt% set by the U.S. Department of Energy (DOE). This work might provide an important theoretical insights for the design, construction and practical application of efficient hydrogen storage materials.