Precipitation will be changed by elevated atmospheric CO_2 concentration in the future, which will seriously affect on the desert ecosystem. Our objective was to determine the interactive effects of elevated CO_2 concentration and changing precipitation on photosynthetic physiology of Reaumuria soongorica which is dominant species of desert, in order to evaluate the response of desert ecosystem to future climate change. Open top chambers were used to simulate the elevated CO_2 concentration (350, 550, and 700 mumol · mol~(-1)) and changing precipitation (30%, 15%, 0,+15%, +30%). The photosynthetic physiology indexes such as net photosynthetic rate, transpiration rate, stomatal conductance and water use efficiency of R. soongorica seedlings were measured in June, July and August. The results showed that, with increased CO_2 concentration, net photosynthetic rate of R. soongorica increased obviously whether precipitation increased or decreased, which meant that the photosynthetic capacity of R. soongorica had a wide adaptability to precipitation under short elevated CO_2. But photosynthetic capacity of R. soongorica decreased under long elevated CO_2 and photosynthetic acclimation occurred during August. Transpiration rate and stomatal conductance decreased with increased CO_2 concentration, but the results were opposite with increased precipitation because of the compensation. Wateruse efficiency increased significantly with increased CO_2 concentration, especially under decreased precipitation treatment, but this reaction was smaller under long elevated CO_2 than under short elevated CO_2. The results suggest that at ambient CO_2 levels, photosynthetic ability of R. soongorica increases with precipitation increasing, and drought resistance increases with water use efficiency increasing under decreased precipitation, which enhance the ability of R. soongorica to climate warming and rying in the future.