Abstract: The effects of various levels of NaCl stress [0 (control), 50, 100, 150, 200, 250, 300 mmol/L] on the osmotic adjustment matters and photosynthetic physiology of Melilotoides ruthenica seedlings were investigated in pot experiments. Soluble protein and sugar contents of seedlings decreased with increasing NaCl concentration. However, proline content increased with increased NaCl concentration. The contents of soluble protein, sugars, and proline differed under different salt concentrations and stress exposure times. On the 7^th day and 14^th day after imposing stress, soluble protein contents of seedlings exposed to 300 mmol/L NaCl concentrations were significantly lower than those of controls (P<0.05). On the 7^th, 14^th and 21^st day after imposing stress, soluble sugar contents of seedlings exposed to 50 or 100 mmol/L NaCl concentration were higher than those of the controls, while on 28^th day after stress, soluble sugar contents of seedlings exposed to 250 or 300 mmol/L NaCl concentrations were significantly lower than those of the controls (P<0.05). On the 7^th day after imposing stress (150-300 mmol/L NaCl), the 14^th day after imposing stress (200-300 mmol/L NaCl) and on the 28^th day after imposing stress (250 or 300 mmol/L NaCl), the proline content was significantly higher than that of the control and other treatments (P<0.05). Hence under salt stress, three indicators of osmotic adjustment had different time signatures. With increases in both NaCl concentration and stress exposure time, the physical damage to M. ruthenica seedlings became more serious, while photosynthetic parameters varied with NaCl concentration and stress exposure time. On the 14^th day after imposing stress, photosynthetic rate (P_n), transpiration rate (T_r), stomatal conductance (Gs) and intercellular CO₂ concentration (C_i) increased at low, and decreased at high salt concentration. P_n, T_r, and G_s of seedlings under 50-100 mmol/L concentrations were significantly higher than those of control plants, while P_n and T_r of seedlings under 300 mmol/L were significantly lower than those of control plants and of other treatments (P<0.05). On 28^th day after imposing stress, P_n, T_r, G_s and C_i were decreased with increasing of salt concentrations. On the 14^th and 28^th days after imposing stress, stomatal limitation (L_s) and water use efficiency (WUE) increased with increasing NaCl concentration.