Using phytotron to simulate high temperature environment, the physiological characteristics of leaves of Simmondsia chinensis seedlings from different provenances (Z₁: kehe in China, Z₂: Condobolin in Australia, Z₃: Phoenix in USA) under different degree of high-temperature stress (CK, T ₁, T ₂) have been studied. The results showed that with the increase of temperature, the leaf relative water content(LRWC), net photosynthetic rate(Pn), transpiration rate (Tr), chlorophyll (Chl) content, and sugar content decreased significantly. While the contents of praline (Pro), malondialdehyde (MDA), and relative conductivity (REC) increased significantly, with the enhancement of temperature stress. However, the effects of high-temperature stress on peroxidase (POD) and superoxide dismutase (SOD) activities were not consistent among different provenances. POD activities of Z₁ and Z₂ increased with the increase of temperature stress intensity, while that of Z₃ first increased and then decreased. SOD activities of Z₁ and Z₃ increased with the increase of temperature stress intensity, however that of Z₂ first decreased and then increased. The photosynthesis mechanisms of S. chinensis were different under diverse high-temperature stress. In high-temperature environment (T ₁), a part of plant stomata closed to reduce the water loss and maintain normal physiological functions. Extreme heat- enviroment (T ₂) broke the systems of ros generation and elimination, and further led ros abundantly accumulating, light oxidation, mda aggravation, plant cell dehydration, chl contents reduction, and the damage of photosynthetic organization. All of these changes brought about by high-temperature stress induced the decrease of Pn of S. chinensis greatly. Comprehensive evaluation of high-temperature resistance of S. chinensis seedlings of three provenances was conducted using fuzzy subordination method. The order of high-temperature resistance ability (from strong to weak) was Kehe in China (Z₁), Phoenix in USA (Z₃), and Condobolin in Australia (Z₂). The results suggest that Z₁ can adapt to the ecological environment of the arid-hot valley of Jinsha River. Z₁ has a better capacity of high-temperature resistance after breeding and cultivation for decades. The reasons that different provenances have diverse high-temperature resistance abilities may be the combined impact of environmental factors on plant, and the adaptation of plant to environment in the long-term of evolution. In short, S. chinensis has a good capability of high-temperature resistance and can be cultivated in extremely hot and dry area. Among the three provenances in the present study, Z₁ has a better ability to adapt to a high-temperature environment, such as the region of Jinssa River valley.