Salt stress is one of the major environmental factors affecting plants; it causes many changes in plant metabolism, such as changes in the content and composition of chlorophyll and reduction in the capacity and efficiency of photosynthesis. Salt stress also inhibits biomass accumulation. Therefore, plant biomass, photosynthesis, and chlorophyll content are often used as indicators of salt stress injury in plants. Ulmus pumila L. is a widely distributed and important fast-growing hardwood timber species that has good tolerance to drought, cold, salt, and wind. It is the most important timber species in China's Northern regions, saline-alkali areas, and desert, where it is used for afforestation, and to provide shelter forest and greenery.Consequently, it is considered to be a promising plant for cultivation in saline land. In response to salt stress, plants have evolved diverse mechanisms that can mitigate the effects of stress and lead to improved plant tolerance. In this study, 1-year-old potted seedlings of Ulmus pumila L. strains (Y65, Y1, and Y34) were used to examine their changes in chlorophyll content, photosynthetic characteristics, and biomass accumulation under different concentrations of NaCl (CK, 50 mmol/L, 85 mmol/L, 120 mmol/L, and 155 mmol/L). The results showed that the content of chlorophyll and biomass of three the Ulmus pumila L. strains were significantly inhibited under higher-concentration salt stress, whereas no obvious changes were observed for the biomass accumulation of Y65 under lower concentrations of NaCl. Under low salinity stress (NaCl ≤85 mmol/L), the values for leaf photosynthetic rate (Pn), stomatal conductance (Gs), and intercellular CO₂ concentration (Ci) were decreased, whereas stomatal limitation (Ls) was increased. Stomatal inhibition (or stomatal restriction) was the main cause of the reduction in Pn of Ulmus pumila L. strains. Although no stomatal restriction occurred in the 120 mmol/L and 155 mmol/L NaCl treatments, the values for Pn and Ls were decreased, whereas Ci was increased. Under low-salinity stress (NaCl ≤85 mmol/L), the increase in Ls resulted in a decrease in transpiration rate (Tr) and an increase in water use efficiency (WUE) that could reduce the leaf water loss, which alleviated the imbalance between water supply and demand and improved the salt-tolerance of Ulmus pumila L. strains. Under high-salinity stress (NaCl > 85 mmol/L), the absorption capacity of roots was decreased, and both leaf Ls and WUE decreased, which resulted in a more severe water imbalance. Correlation analysis indicated that the biomass of Ulmus pumila L. strains was highly significantly positively related to total chlorophyll, chlorophyll a, Pn, Gs, and significantly positively related to Tr. Furthermore, WUE was significantly positively related to Ls. Collectively, our results indicated that the salt resistance of Y65 was better than that of Y1 and Y34. We accordingly recommend that Y65 is the preferred Ulmus pumila L. strains for afforestation in saline areas, which could provide a scientific basis for further research on the salinity tolerance mechanism.