Photosynthesis of Saposhnikovia divaricata (Turcz.) Schischk. and environmental factors were measured by using LCPro+ portable photosynthesis system in Changchun, Changling and Baicheng, the diurnal changes in net photosynthetic rate (P_n), transpiration rate (E) and water use efficiency (WUE) of S. divaricata and their relationships with environmental factors were studied, and direct and indirect effects of environmental factors on net photosynthetic rate were calculated through the path analysis in different habitats in order to discuss photosynthetic physiology rule of S. divaricata . This knowledge could provide basis for further study the photosynthetic physiology of S. divaricata . The results showed that there were significant interactions among environmental factors, variation of photosynthetically active radiation (PAR) triggered the fluctuations of air temperature (T_a) and leaf temperature (T_L), and T_a was closely related to the air density, which had significant impacts on the air CO₂ concentration (C_a) and relative humidity (RH), the reduce of air density affected by increase of T_a was the one of important causes, which decreased the RH and C_a at noon. Environmental factors driven intrinsically by PAR superposed an integrative effect on P_n. And the responses of the species to the environmental factors were different in different habitats, and the direct effects of environmental factors in three different habitats were of the order: T_l >T_a > PAR >C_a >RH in Changchun, PAR >T_l >RH >C_a >T_a in Changling and T_a >T_l >RH > PAR >C_a in Baicheng. The correlation coefficients between PAR and P_n were the biggest in Changchun and Changling, but C_a and P_n was the biggest in Baicheng. The diurnal changes in P_n of the species in different habitats were similar and could be expressed as two-hump curves, reflecting a significant middy depression at noon. The decrease in P_n during midday was mainly resulted from stomatal limitation in Changchun and Changling, but non-stomatal limitation occurred in Baicheng. There were significant positive correlations between P_n and G_s in three ecological regions, G_s in Baicheng was about 44% lower than in Changchun and Changling, this was a physiological reaction in order to response to high temperature and drought in Baicheng. The soil water content differed significantly in different habitats, it was only 20.35% in Baicheng, and that was 56.50% of that in Changchun, but air temperature in Baicheng was high, leaf water content was only 56.60%, which resulted from leaf strong transpiration and water loss affected by intense radiation. So the net photosynthetic rate decreased because of that the phosphorylation, electron transfer and enzyme activities of S. divaricata were restrained by high leaf temperature. So heat-stress was one of critical factors constraining P_n in Baicheng.