The i-SANEX process has been developed as a promising strategy for the separation of trivalent lanthanide-actinides, which can effectively overcome the poor solubility of N-donors in nonpolar solvents. In this study, four hydrophilic phenanthroline-based ligands (L1-L4) with different terminal groups were synthesized to evaluate their water solubility and masking effects on An(III) in aqueous solutions using competitive extraction dominated by TODGA. Among these ligands, BHEO-DAPhen (L1), featuring an ethoxyethanol-modified side chain, demonstrated the best overall performance in both solubility and selectivity. Competitive two-phase extraction experiments confirmed that L1 significantly enhanced the separation factor (SFEu/Am), reaching a striking maximum value of 558 in 0.6 M HNO3. The coordination behavior of L1 with trivalent metal ions was comprehensively investigated using UV-visible titration, 1H NMR titration, IR spectroscopy, ESI-HRMS, TRLFS spectroscopy, and X-ray single-crystal diffraction. DFT calculations also revealed that the exceptional selectivity of L1 toward Am(III) is attributed to stronger covalent interactions than Eu(III). These results demonstrated the potential of L1 as a promising masking agent for the selective separation of An(III) and Ln(III), providing valuable insights for the treatment of next-generation nuclear wastes.