The adsorption properties of water molecules on an MgSO4(100) surface were investigated by using density functional theory (DFT) and supercell models. Optimized stable geometries of one and more than one water molecules adsorbed on an ideal MgSO4(100) surface were obtained. The configurations with water molecules adsorbed on atoms of the second and third atomic layers of the MgSO4(100) surface are quite stable. After adsorption, the separations between both the adjacent Mg atoms (RMgMg) and the adjacent O atoms of the surface (ROO) increase, which indicates that the MgSO4(100) surface starts to deliquesce. In addition, water molecules are more likely to adsorb onto a defective surface rather than an ideal surface. Mulliken population analysis suggests that fewer charges transfer to the water molecule from the Mg atom of a defective substrate. Finally, Raman spectra were calculated for 0.5, 1, and 2 ML (ML=monolayer) water adsorbed on an MgSO4(100) surface, which is helpful for further related experiments.