Abstract: Salt-alkali stress is a major constraint on maize (Zea mays L.) production in arid and semi-arid regions, yet the differential vulnerability of functionally distinct root modules to this stress remains poorly understood. This study investigated the anatomical basis underlying root-order-specific sodium accumulation and functional decline under salt-alkali conditions. A field experiment was conducted using the maize hybrid Xianyu 335 on typical salt-alkali farmland in Alar, Xinjiang, China. Roots were separated into five hierarchical orders (I～V) using a root-order classification technique, and ion concentrations, anatomical traits, plasma membrane integrity, and root lifespan were quantified for each order. Results revealed pronounced root-order heterogeneity in Na⁺ accumulation under salt-alkali stress: first-order absorptive roots exhibited Na⁺ concentrations 3.0-fold greater than those of third-order transport roots, with the K⁺/Na⁺ ratio declining to 0.32. Anatomical divergence among root orders was identified as the structural foundation for this differential ion loading. First-order roots possessed thinner cortical layers and markedly lower lignification levels; cortical thickness and lignin content together accounted for 78% of the inter-order variance in Na⁺ concentration. The physiological consequences of salt-alkali stress were likewise root-order-specific: electrolyte leakage in first-order roots increased by 222% and lifespan was shortened by 47.4%, whereas fourth- and fifth-order transport roots showed only a 25% increase in leakage and a 12.0% reduction in lifespan. These findings demonstrate that first- and second-order absorptive roots, owing to their weak ion-exclusion barriers, serve as preferential sites for Na⁺ accumulation, and their accelerated functional deterioration constitutes a critical pathway of salt-alkali injury. This work provides a theoretical basis for shifting salt-alkali land amelioration strategies from simply "increasing total root biomass" toward "preserving absorptive root functionality," and supports the development of precision management approaches targeting specific root functional modules.