In order to explore the effect of exogenous MT on the growth and physiological mechanism of Toona sinensis seedlings under salt stress, we studied the effects of different concentrations of melatonin (0, 50, 100, 200 and 400 μmol/L) on the growth indicators, mineral ion (Na⁺, K⁺, Ca²⁺ and Mg²⁺) contents and net photosynthetic rate (P_n), transpiration rate (T_r), stomatal conductance (G_s) and intercellular CO₂ concentration (C_i) of T. sinensis seedlings under 150 mmol/L NaCl. The results showed that: (1) compared with the control (CK), the growth of T. sinensis seedlings was significantly inhibited by salt stress and had a lower P_n. 150 mmol/L NaCl significantly increased Na⁺ contents in leaves and roots and reduced the contents of K⁺, Mg²⁺ and Ca²⁺ with the decreased values of K⁺/Na⁺, Mg²⁺/Na⁺ and Ca²⁺/Na⁺. (2) Exogenously suitable concentration of MT increased the plant growth indexes of seedlings under salt stress, decreased Na⁺ contents, increased the levels of K⁺, Ca²⁺ and Mg²⁺ and values of K⁺/Na⁺, Mg²⁺/Na⁺ and Ca²⁺/Na⁺. At the same time, P_n, T_r, water use efficiency (WUE), G_s, C_i and decreased stomatal limitation value (L_s). (3) The chlorophyll contents increased and MDA content decreased significantly with MT treatment. The results showed that appropriate concentration (100 μmol/L) of MT can reduce the concentration of Na⁺ by increasing the concentration of K⁺, Mg²⁺ and Ca²⁺ in the leaves and roots of T. sinensis seedlings, regulate the ion balance in plants, enhance the absorption of nutrients, and improve the photosynthesis efficiency in order to improve the resistance of T. sinensis seedlings to salt stress.