Abstract: Spent fuel reprocessing generates minor actinides such as Am³⁺ and Cm³⁺. Owing to their long half-lives and high radiotoxicity, their efficient separation represents a key issue in nuclear chemistry and environmental remediation. Eu³⁺, with physicochemical properties highly analogous to Am³⁺ and Cm³⁺, is frequently employed as a non-radioactive surrogate to investigate their separation mechanisms. In strongly acidic and multicomponent systems, the efficient and selective immobilization of trivalent minor actinides remains challenging, with uncontrollable interfacial reaction pathways as the major bottleneck. However, in real high-level liquid waste systems characterized by strong acidity, high salinity, and multi-component coexistence, conventional materials frequently suffer from bottlenecks such as poor structural stability and rapid passivation of active sites. These drawbacks render the efficient and selective immobilization of trivalent minor actinides extremely difficult, a limitation primarily rooted in the uncontrollable nature of complex solid-liquid interfacial reaction pathways. To address this challenge, in this study, phosphate-modified zero-valent iron(P-ZVI_bm) was fabricated, and a dynamic reactive interface was constructed to achieve efficient separation of Am³⁺ and Cm³⁺ using Eu³⁺ as a surrogate under acidic conditions. The Fe−O−P interface induced by phosphate regulates interfacial electron behavior, drives the separation mechanism from solution-dominated to interface-dominated, and facilitates inner-sphere complexation of Eu³⁺ and interfacial phosphate phase transformation. Spectroscopic characterizations combined with DFT calculations verify that Eu³⁺ forms stable Eu−O−P hybrid bonds at the Fe−O−P interface via multidentate coordination, with an adsorption energy as low as −14.7 eV. The 4f and 5d orbitals of Eu are strongly hybridized with the 2p orbital of O, revealing characteristic strong chemical bonding. Meanwhile, such interfacial regulation significantly improves the structural stability of zero-valent iron in acidic media. In complex multicomponent wastewater systems, P−ZVI_bm exhibits high selectivity toward Eu³⁺ and excellent reusability, with a desorption efficiency maintained above 85% after multiple cycles. This study demonstrates that phosphate-mediated interfacial electron regulation provides a feasible strategy for the efficient separation of trivalent f-block elements, offers theoretical support for interface-driven separation chemistry, and presents a new approach for the resource recovery and pollution control of minor actinides during spent fuel reprocessing. It also provides a novel technical scheme with significant potential for engineering applications in the resource recovery, volume reduction, solidification, and environmental safety control of minor actinides in high-level liquid waste from China’s future spent fuel reprocessing plants.