Intraspecific variation in plant traits objectively reflects plant adaptability, making the study of phenotypic traits and seed germination characteristics of different geographic populations highly significant in the fields of ecology and plant evolution. Leymus chinensis, a dominant native species in the Songnen Plain of China, frequently forms monodominant communities. However, existing research has largely focused on its clonal reproductive traits, with fewer studies addressing seed germination and seedling growth during critical stages in different populations. In this study, seeds from 14 populations of L. chinensis (P01—P14) in the saline-alkaline grasslands of the western Songnen Plain were selected to analyze their phenotypic traits and seed germination characteristics. Germination characteristics and seedling growth responses were assessed under different NaCl concentrations (0, 50, 100, 200, and 400 mmol/L), aiming to explore the relationship between seed phenotypic traits, salt tolerance, and environmental factors in the seed source regions. The results showed that with increasing salt concentrations, the germination rate and germination index of seeds from all populations exhibited a declining trend. Apart from populations P13 and P14, seed germination in the remaining 12 populations was significantly inhibited at NaCl concentrations above 100 mmol/L, and no seed germination was observed in 9 populations at 400 mmol/L NaCl. Regarding seedling growth, the seedling length initially increased and then decreased with increasing salinity, while root length was more sensitive to salt stress, showing significant inhibition at as low as 50 mmol/L NaCl. However, the number of roots increased with salt concentration, demonstrating a high degree of plasticity in root development. Moreover, seeds that failed to germinate under salt stress were capable of rapid germination upon removal of the stress. Except at 100 and 200 mmol/L NaCl, the total germination rate after stress recovery was not significantly different from the control, indicating that L. chinensis seeds possess strong salt tolerance. Some seeds entered secondary dormancy under salt stress to maintain viability. Based on a comprehensive salt tolerance evaluation (D-value) combining seed germination and seedling growth indices, the ranking of populations was as follows: P13 > P14 > P02 > P01 > P09 > P07 > P11 > P05 > P12 > P03 > P04 > P08 > P06 > P10. Correlation analysis and stepwise regression showed that mean annual temperature and mean annual precipitation were significantly associated with seed surface area and caryopsis length. The comprehensive salt tolerance evaluation (D-value) was primarily influenced by seed caryopsis surface area and soil pH at the sampling sites. Populations from regions with higher soil pH and larger seed sizes exhibited better adaptation to salt stress. In summary, there were significant differences in seed phenotypic traits, germination, and seedling growth characteristics under salt stress among different L. chinensis populations. The climate and environmental conditions of the seed source regions are likely the main drivers of these differences. Salt stress prompted some seeds to re-enter dormancy, while low salt concentrations promoted the growth of germinated seedlings and root development. These findings highlight important adaptive mechanisms of L. chinensis seeds to saline-alkaline environments, providing scientific evidence for further understanding the adaptation strategies and evolutionary mechanisms of L. chinensis populations in such habitats.