To explore differences in the physiological metabolic response mechanisms of grassland perennial plants to different intensities of salt–alkali stress, we employed GC-MS to identify the metabolome of perennial rhizome-saline-tolerantLeymus chinensis(L. chinensis).L. chinensisreduced stress damage by accumulating osmotic solutes during salt–alkali stress, although the types of accumulated solutes varied with stress and concentration gradients. Soluble sugars increased only under mild salt–alkali stress. Under salt and mild alkali stress, amino acids increased. Under severe salt–alkali stress, organic acids increased. Betaine increased as a typical osmolute under salt–alkali stress. Metabolic analysis identified 20 metabolites, including 4 amino acids, 6 sugars, and 10 organic acids. The majority of them increased in response to stress. Under mild salt stress, the metabolites included glycine and proline. Under mild alkali stress, they primarily consisted of sugars such as isomaltose and lactulose, whereas under severe salt–alkali stress, they primarily consisted of organic acids such as citric acid and isocitric acid. Pathway analysis showed that six pathways were affected. Glycine, serine, and threonine metabolism was affected under mild salt stress. Alanine, aspartate, and glutamate metabolism and butanota metabolism were affected under mild alkali stress, while energy metabolism pathways, such as the TCA cycle and glyoxylate and dicarboxylate metabolism, were affected under severe salt–alkali stress. The results indicate the importance of betaine in stress resistance and the significance of organic acid in severe salt stress, and they also demonstrate that energy supply was one of the key mechanisms in response to severe salt–alkali stress.