Abstract: To explore the photosynthetic physiological response of forage to alkaline stress, ryegrass (Lolium perenne) seedlings grown in sands in a greenhouse were exposed to NaHCO₃ at concentrations of 0, 50, 100, 150, or 200 mmol/L in Hoagland’s nutrient solution. The effect of NaHCO₃ stress on the photosynthetic pigment content, gas exchange and chlorophyll fluorescence parameters, and the xanthophyll cycle in the leaf was assessed.1) With an increase in NaHCO₃ concentration, the chlorophyll and carotenoid contents decreased gradually, but the chlorophyll a/b increased continuously. Net photosynthetic rate (P_n), stomatal conductance, transpiration rate, and stomatal limitation to P_n of ryegrass leaves decreased, but the intercellular CO₂ concentration increased, suggesting that non-stomatal limitation was the main cause of the P_n decrease under alkaline stress. 2) The initial fluorescence and non-photochemical quenching (NPQ) increased with increasing concentration of NaHCO₃, while the maximum fluorescence value (F_m), maximal photochemical efficiency (F_v/F_m), actual yield (Φ_PSⅡ), and photochemical quenching of PSⅡ decreased significantly. 3) With an increase in NaHCO₃ concentration, the antenna efficiency at open centers in the presence of NPQ decreased, the deviation from full balance between PSI and PSⅡ improved, the absorbed light energy allocated to the photochemical reaction of PSⅡ reduced, while the proportion of energy lost during antenna heat dissipation increased significantly. The de-epoxidation extent of the xanthophyll cycle (A+Z)/(V+A+Z) initially increased but then decreased, suggesting that antenna heat dissipation dependant on the xanthophyll cycle was the main mechanism for excessive energy dissipation.