Background and aims <p>Soil salinity is a major constraint on forage crop production, especially in arid and semi-arid regions. Alfalfa (<i>Medicago sativa</i> L.) is widely cultivated for its high nutritional value, but its productivity and metabolic stability are challenged under salt stress. This study aimed to explore how salt stress influences alfalfa's amino acid metabolism and nutritional quality using field-based and metabolomic approaches.</p> Methods <p>A two-factor field experiment was conducted using saline-alkali and non-saline soils. Two alfalfa crops harvested at the early flowering stage (May and July) were analyzed for nutritional components, amino acid profiles, and protein fractions. Untargeted metabolomic profiling via UHPLC–QTOF–MS was used to identify differential metabolites and enriched pathways related to salt stress.</p> Results <p>Salt stress significantly increased the contents of crude protein, soluble protein, and several amino acids including proline, serine, and methionine, while inhibiting biomass accumulation. A total of 10 key differential metabolites were identified, with enrichment in amino acid biosynthesis, glycolysis, and ABC transporter pathways. Na⁺ accumulation altered central carbon metabolism, promoting glycolysis and amino acid synthesis while inhibiting the TCA cycle.</p> Conclusion <p>Salt stress induces significant metabolic reprogramming in alfalfa, with amino acid pathways playing a central role in osmotic adjustment and nutritional enhancement. These results provide mechanistic insights into alfalfa's salt response and suggest strategies for improving forage quality in salt-affected agroecosystems.</p>