Humic acid is an important active component in soil environment.The spatial structures of P complexation sites in humic acid complexes play an important role in soil phosphorus activation and fertilizer efficiency.To explore the effects of spatial structure,the three different coordination modes of iron-carboxyl in models were calculated by the ONIOM method available in the Gaussian09 package.The (U)B3LYP hybrid density functional was employed to optimize the configuration for the QM region,and the UFF force field was used to calculate for the MM region.The results show that the different spatial structures influence the soil phosphorus activation by affecting the electronic structure,Gibbs free energy and interaction energy of the models.And the effects are as follows:the unidentate structure model 6p-Fe-MHA-UD,the bidentate chelating structure model 6p-Fe-MHA-BD＞the bidentate bridging structure model 5P-Fe-MHA-BD-BG.It can be known that,the fertilizer efficiency can be improved through increasing the proportion of the unidentate structure and the bidentate chelating structure in production engineering.The research provides a theoretical basis for further optimization of the production of humic acid phosphate fertilizer.