H2 has been comprehensively deemed a promising potential candidate to replace traditional fossil fuel-based energy. Typically, the hydrolysis of most hydrogen-rich boron hydrides (e.g. NaBH4, NH3BH3 and Me2NHBH3) catalyzed by nanomaterials generates H2 with only one H atom supplied by water and the other one by a hydrogen-rich boron hydride. Interestingly, both H atoms of produced H2 are provided by water upon hydrolysis of B2(OH)4. Herein, the catalytic mechanisms of H2 evolution upon water splitting at the expense of B2(OH)4 in its hydrolysis reactions catalyzed by acid, base or metal nanoparticles have been investigated by density functional theory (DFT) calculations. By computational studies, the mechanisms of catalysis by base and metal nanoparticles are basically the same as those speculated from our previous experiments. The previously proposed acid catalytic mechanism has been overturned, however. This study not only provides important insights into the catalytic mechanism for water splitting at the expense of B2(OH)4, but also opens up an exciting opportunity to use water to store H2.