Abstract: With the intensification of climate change, high temperature poses increasing threats to economic forests. Pinus. massoniana seedlings from two provenances, namely Sanming, Fujian (FJSM) and Jiujiang, Jiangxi (JXJJ), were utilized as objects to investigate the effects of high temperature stress on the gas exchange and chlorophyll fluorescence characteristics. Four temperature gradients (25 ℃, 30 ℃, 35 ℃ and 40 ℃) were set up, gas exchange parameters such as net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO₂ concentration (Ci), as well as fast chlorophyll fluorescence (OJIP) curve, delayed fluorescence (DF) curve and 820 nm reflection (MR/MR₀) were measured in the experiment. The results showed that the decrease in Pn and Gs of FJSM was relatively small under high temperatures (35 ℃ and 40 ℃), and the changes in OJIP and DF curves were also minor, indicating that its photosystem II and photosynthetic electron transfer had high stability against heat stress. In contrast, under high temperatures the Pn and Gs of JXJJ dropped significantly, and the OJIP and DF curves fluctuated to a greater extent, indicating its stronger sensitivity to high temperatures. In addition, the oxidation rates (V_ox) of MR/MR₀ and P₇₀₀ in FJSM showed greater tolerance to high temperature, whereas those in JXJJ decreased obviously. In conclusion, seedlings from Sanming, Fujian (FJSM) exhibits greater stability in photosystem II and higher photosynthetic efficiency under high-temperature stress, indicating stronger heat adaptability. This study provides scientific foundation for selecting heat-resistant provenances of P. massoniana under high temperature condition.