With the depletion of fossil fuels and the environmental problems, the development and utilization of new energy resources is imminent. Hydrogen energy is one of the main new energy sources in the 21st century. Finding stable and efficient hydrogen storage materials is the key to achieving the hydrogen economy. Transition metal (TM)-decorated graphenes have been widely studied as hydrogen storage materials theoretically, but they suffer metal agglomeration and H ₂dissociation. Our calculations show that the reconstruction energy of Sc, Ti, V decorated pristine graphenes in the process of adsorption and desorption of hydrogen molecules are only 0.00, 0.12 and 0.08 eV, respectively. The adsorption energy values of the first H ₂dissociation adsorption on the Sc, Ti, V decorated pristine graphenes are –1.34, –1.34, and –1.16 eV, respectively. So, some hydrogen molecules are difficult to desorb at room temperature and medium pressure. In this paper, the stability and hydrogen storage properties of Sc, Ti, V decorated monovacancy graphene are also investigated based on density functional theory. The results show that the binding energy values between Sc, Ti, V and themonovacancy graphene are –6.93, –8.82, –9.30 eV, respectively, which indicate monovacancy can effectively avoid metal aggregation. The Sc, Ti and V atoms decorated on the monovacancy graphene would transfer more electrons to the carbon material with charge of +1.24| e|–+1.37| e|. They can adsorb 7, 3 and 4 hydrogen molecules through electrostatic interaction. When a monovacancy is introduced, all of the hydrogen molecules are adsorbed in molecular form. The average adsorption energy values of H ₂are –0.13, –0.20 and –0.18 eV, respectively, which are in the best energy range for the adsorption/desorption process at room temperature and medium pressure. The most important thing is that their deformations in the adsorption/desorption process are very small, which is conducive to the rapid hydrogen adsorption/desorption. The calculated results show that the monovacancy introduction can effectively solve the two major problems, i.e. metal agglomeration and hydrogen molecular dissociation during hydrogen storage on Sc, Ti, V decorated pristine graphenes. The research in this paper will be helpful to further understand the hydrogen storage mechanism of 3d TM-decorated carbon nanomaterials.