Jinsha river dry-hot valley with a fragile ecology in Southwest China is characterized by hot and dry climate and serious vegetation degradation, that is difficult to rehabilitate the original ecosystem structure and functions. In order to study the adaptability of the introduced tree species to environment of Jinsha river dry-hot valley, photosynthetic physiological characteristics and chlorophyll fluorescence characteristics of Leucaena leucocephala were measured in dry season (November)and wet season (July), respectively. The results showed that:(1) According to the P_n-C_i response curves,net photosynthetic rate (P_n) in dry season was higher than that in wet season at C_i < 150 μmol/mol, but it was lower at C_i > 150 μmol/mol. (2) Compared with wet season, vapor pressure deficit (VPD), stomatal limitation (L_s), initial carboxylation efficiency (CE), photosynthetic capacity (A_max), CO₂ compensation point (Γ), photorespiration rate (R_p), the maximum carboxylation rate (V_cmax), the maximum electron transport rate (J_max), triose phosphates utilization rate (TPU), J_max/V_cmax, actual photochemical efficiency of PSII (Ф_PSII), photochemical fluorescence quenching (qP), and electron transport rate (ETR) of Leucaena leucocephala decreased significantly in dry season (P < 0.05). (3) Compared with wet season, intercellular CO₂ concentration (C_i), stomatal conductance (G_s), transpiration rate (T_r), water use efficiency (WUE), light-saturated net photosynthetic rate (P_nmax), dark respiration rate (R_d), and non-photochemical fluorescence quenching (NPQ) of Leucaena leucocephala significantly increased in dry season (P < 0.05). (4) Ф_PSII, NPQ and ETR, three indices of chlorophyll fluorescence parameters, well correlated with photosynthetic physiological indices. In conclusion, the main factor influencing the photosynthetic rate of Leucaena leucocephala was the stomatal limitation in wet season, but it was non-stomatal limitation in dry season.