Abstract: In this research, the possibility of exogenous application of 5-aminolevulinic acid (ALA) on photosynthetic characteristics of tomato seedlings under NaCl stress was investigated. Five leaves seedlings of tomato (Solanum lycopersicum cv. Jinpeng No.1) were used as starting materials, applied with 50 mg·L^-1 ALA by foliage spray or 10 mg·L^-1 ALA by root soaking to study the changes in their photosynthesis and fluorescence parameters under 100 mmol·L^-1 NaCl. The result showed that, photosynthetic gas exchange parameters (net photosynthetic rate P_n, stomata conductance g_s, intercellular CO₂ concentration C_i,
transpiration T_r) and chlorophyll fluorescence parameters (F_v′/F_m′, F_m′, Φ_PSⅡ, ETR, q_P, Pc) were severely reduced under NaCl treatment and ALA application by foliage spray or root soaking with proper concentrations exerted positive influences on tomato seedlings under salt stress, while there were some differences between foliage spray and root soaking in the influence on chlorophyll content, photosynthesis and chlorophyll fluorescence. Both foliage spray with 50 mg·L^-1ALA and root soaking with 10 mg·L^-1ALA significantly increased  P_n, C_i, g_s and  T_r of tomato seedlings under NaCl stress, alleviated photosynthetic inhibition. Root application of ALA had a better effect on the chlorophyll content than foliage application. However, the photosynthetic parameters showed that foliage application of ALA had a better effect than root application, and both treatments had no difference in the influence on chlorophyll fluorescence parameters of tomato seedlings. It could be deduced that the regulating effect of ALA on enhancing salt tolerance of tomato seedlings is attributed to its effect on improving chlorophyll biosynthesis and metabolism, increasing stomatal conductance and reducing stomatal limitation, thus, enhancing the photosynthetic capacity and PSⅡ photochemical efficiency of tomato leaves under NaCl stress.