Despite considerable tolerance to salt and alkali stress, Leymus chinensis populations on the southwestern Songnen Plain in northern China are threatened by increasing soil salinity and alkalinity. To explore the species’ responses to saline-alkali stress, we grew it in substrates with varying concentrations of nitrogen (N) and phosphorus (P) while applying varying levels of saline-alkali stress (increasing in 14-, 17- or 23 -day intervals). We measured the plants’ contents of N and P, and the N:P ratio, and calculated their homeostasis indices ( H N , H P and H N:P ) under each nutrient and saline-alkali stress treatment. The N content was found to be more sensitive to saline-alkali stress than the P content. The N and P contents were highest and the N:P ratio was stable at pH 8.4. At both pH 8.1 and 8.4, H N:P > H N > H P , but the indices and their relations differed at other pH values. Exposure to saline-alkali stress for the 14-day incremental interval had weaker effects on the plants. Rapid changes in salinity-alkalinity weakened both the positive effects of the weakly alkaline conditions (pH 7.5–8.4) and the negative effects of more strongly alkaline conditions (pH 8.7 or 9.3) on L. chinensis . When L. chinensis plants lack N, applying N fertilizer will be extremely efficient. The optimal concentrations of N and P appeared to be 16 and 1.2 mmol/L, respectively. When the L. chinensis plants were N- and P-limited, the specific growth rate correlated positively with N:P, when limited by N it correlated positively with the environmental N concentration, and when limited by P it was weakly positively correlated with the environmental P concentration.