The water adsorption on Fe (100) surface with and without surface tensile strain is studied by spin-polarized self-consistent density functional theory calculations to elaborate the influence of tensile strain on surface chemistry. We find that the surface tension not only enhances the interaction between water and surface but also lowers the energy barrier for water dissociation on the surface. The dissociated reactive OH fragment may further attack the iron surface and the dissociated reactive H fragment could transfer into iron lattice and cause H embrittlement. This study clearly shows that the stressed induced surface chemistry could be an important aspect of stress-corrosion cracking (SCC) in iron based materials.