Abstract: The molecular geometry, orbit and steric hindrance of the complexes of Fe(EDTA)^-and Fe(EDTMP)^- were calculated using the B3LYP method of density functional theory. The DEF2-TZVP basis set was used for all atoms and water was used as the solvent. The reasons for the difference in dissolving scale ability were analyzed. The results show that the complex is a low-spin six-coordinate octahedral structure. The binding energy of EDTA to Fe³⁺ ions is greater than that of EDTMP. The delocalized π bond of the carboxylic acid group of EDTA strengthens the electron delocalization of coordination bonds for complexes, which makes the covalent interaction of Fe(EDTA)^- coordination bond stronger than that of Fe(EDTMP)^-, whereas the phosphonic acid group of EDTMP has only σ bond. The steric hindrance effect between phosphonic acid groups of Fe(EDTMP)^- is also not conducive to the combination of EDTMP and Fe³⁺ ion.