Direct　production　of　high-purity　amino　acid　salts　offers　an　efficient　and　sustainable　solution　to　reduce　fresh　acid/base　consumption　and　wastewater　generation.　In　this　study,　we　specifically　designed　a　selective　cation　substitution　electrodialysis　(SCS-ED)　with　a　novel　configuration　to　enable　the　direct　production　of　high-purity　magnesium　L-aspartate　(Asp‐Mg)　from　monoammonium　L-aspartate.　To　achieve　this,　we　firstly　established　an　ion　transport　model　to　quantify　the　ion　transport　including　the　competitive　migration　of　NH4+　and　Mg2+　through　the　selective　cation　exchange　membrane,　and　the　co-ion　transport　of　NH4+　through　the　anion　exchange　membrane.　Then,　effects　of　various　parameters　including　stack　voltage,　feed　concentration,　and　feed　velocity　on　the　ion　transport　as　well　as　SCS-ED　performance　were　investigated　comprehensively.　Results　indicate　that　decreasing　stack　voltage　acts　as　a　significant　role　in　obtaining　a　high　selectivity　(SNH4+/Mg2+SCS−ED)　and　high　purity　(PAsp−Mg),　decreasing　feed　concentration　acts　as　a　significant　role　in　obtaining　a　high　current　efficiency　(ηNH4+SCEM′)　and　SNH4+/Mg2+SCS−ED,　and　increasing　feed　velocity　acts　as　an　important　role　in　obtaining　a　relatively　low　total　coat.　Overall,　considering　the　high　PAsp−Mg　for　pharmaceutical　application,　a　high　SNH4+/Mg2+SCS−ED　(32),　PAsp−Mg　(99.8　%),　and　ηNH4+SCEM′　(91.3　%),　and　a　relatively　low　total　cost　(0.946　$　kg-1)　can　be　achieved　in　SCS-ED　process　at　optimized　parameters.　The　above　findings　give　new　insights　into　SCS-ED　for　directly　producing　high　purity　Asp‐Mg.　Compared　with　traditional　methods,　this　ion　substitution　approach　provides　a　more　efficient　and　streamlined　process,　showing　strong　potential　in　the　production　of　high-purity　divalent　salt　(e.g.,　amino　acid　divalent　salt).　©　2025