The exploitation of new hydrogen storage materials with high-capacity and reversible performance could play a very important role in the large-scale utilization of hydrogen as an energy source. The hydrogen storage performances of Li and Ca atoms decorated as well as Li/Ca co-decorated VO2 monolayer system were comprehensively investigated based on first-principles calculations. The metal atoms are stably adsorbed on the surface of the VO2 monolayer without forming metal clusters. Three and six hydrogen molecules can be absorbed by a Li atom and Ca atom, respectively, and its average adsorption energy is larger than 0.20 eV/H2. Charge density differences and density of states of H2 adsorbed systems were analyzed, and the results reveal that both the polarization mechanism and orbital hybridization are responsible for the adsorption of hydrogen molecules. The hydrogen storage capacity of the Li-decorated system increases with the increase of Li coverage, while that of Ca-decorated system is high only in lower Ca coverage. Moreover, Li/Ca co-decoration can effectively increase the hydrogen storage capacity of the system, its hydrogen storage mass density is 5.00% (mass fraction). Finally, the influence of temperature and pressure on the stability of hydrogen adsorption system is studied.