Diethyl phthalate ester (DEP) can be degraded in Fe3+-UV- aliphatic/phenolic carboxyl acids systems, and the degradation can be significantly influenced by different physical chemical properties of the organic acids. In this study, a density function theory (DFT) approach combined with multilinear regression analysis demonstrates that the lowest unoccupied molecular orbital energy (E-LUMO) significantly negatively influenced DEP degradation. The correlation coefficient of the regression is -19.3 mM L-1 h(-1). The standardized partial correlation coefficient is -0.674. The aliphatic acids group has higher catalytic ability than the phenolic carboxyl acids group, and by shifting the dominant LUMO configuration from C-3s3d to Fe-4s3d orbitals, Fe3+ significantly accelerates the catalytic reactions of the aliphatic acids/UV systems. Further analysis demonstrates that electron withdrawing hydroxyl group on the aliphatic acids increases the photo catalytic effects of aliphatic acids/UV system. This study highlights the important role of E-LUMO of organic acids on their photocatalytic ability towards degradation of organic pollutants.