Ab initio molecular dynamics (AIMD) simulations were performed to study the adsorption of the 1-hydroxyethane-1,1-diphosphonic acid (HEDP) molecule on the Fe (100) surface. Through molecular dynamics trajectory, changes in bond length and angle, density distribution, interaction region indicator (IRI) and electron localization function (ELF), the fundamental adsorption mechanism of HEDP on the iron surface was disclosed in detail. The oxygen atoms of four phosphonic acid groups of HEDP coordinated with the four Fe atoms form a stable adsorption on the iron (100) surface. The HEDP molecule is slightly distorted to bind with O atoms after adsorption. The bond length change tracking results show that the adsorption occurs very quickly. IRI analysis and Bader charges show that the electrostatic interaction between HEDP and the iron surface is strong and responsible for the stable adsorption state. The interaction between deprotonated O atoms and iron surface is stronger than that between protonated O atoms and iron surface.