The adsorption of H2O on Fe（100）, Fe（110） and Fe（111） crystal surfaces were studied by first principles. The calculation results indicate that H2O monomers bind preferentially at top sites and lie nearly flat on the three crystal surfaces. The results of adsorption energy and geometry structure show that the effect of H2O adsorbed on three iron crystal surfaces is different. The strongest interaction occurred between H2O and Fe（111） crystal surface and the weakest interaction is between H2O and Fe（110） crystal surface. Such relationship is related to the different surface atoms density. The similar conclusion can be drawn from the calculation results of electronic structure and Mulliken analysis. The Mulliken analysis also indicates that when H2O is adsorbed on the iron surface, charge exchange between O atom and Fe atom make the iron surface negatively charged and reduce the surface potential, which promote the electrochemical corrosion of iron surfaces.