The photosynthetic characteristics and their control factors of Reaumuria soongorica from three geographical populations (LZJ, WWM, and ZYL) were studied under natural environment. The results showed that diurnal variations in net photosynthetic (Pn) rate and transpiration rate (Tr) of R.soongorica from different populations displayed a similar pattern——a bimodal curve but with an obvious depression in Pn, occurring in midday. Both environmental factors and internal biological causes of the plants played roles in controlling Pn and Tr. Among the environmental factors, photosynthetically active radiation (PAR) had a leading effect on both Pr and Tr while air temperature (Ta) affected less. Relative humidity (RH) affected more onTr than on Pn. It was also noted that the effects on Pn and Tr varied with the water contents in the soil where the plants grew. Among internal factors, stomatal conductance (Gs) was the major determinant of Pn which was concurrently determined by Tr that was affected by environmental factors via Gs. Photoinhibition which could be aggravated by water stress was also responsible for the midday depression in Pn. Chlorophyll was an additional factor affecting Pn under water stress condition. Although the mentioned factors may interactively influence on the photosynthesis, R.soongorica growing in different environments, in particular with super water stress, may adapt the adverse environments through adjusting their transpiration rates, increasing water use efficiency(WUE) and δ13C.