Atmospheric CO₂ concentrations are predicted to increase from approximately 350μmol/mol today to over 700μmol/mol in the late 21th century. In the future, elevated CO₂ levels are likely to have profound effects on precipitation.This change would seriously affect the desert ecosystem, above-and belowground biomass, and carbon and nitrogen allocations of desert plants, leading to changes in ecosystem structure and function. Although many studies have examined the effects of precipitation and CO₂, the interactions between changing precipitation and CO₂ on desert plants have attracted little attention to date. A pot experiment was conducted to study the interaction of elevated CO₂ concentration and changing precipitation with biomass allocation, and carbon and nitrogen content characteristics in roots, stem, and leaf of Reaumuria soongorica, a dominant species of the desert steppe in the arid region of China, in order to assess the possible effect of global climate change on desert ecosystems. The main plot included two CO₂ concentrations (350 and 700μmol/mol) and five precipitation conditions(natural precipitationas control[0], precipitation minus 30%[-30%], precipitation minus 15%[-15%], precipitation plus 15%[15%], precipitation plus 30%[30%]). The results showed that:(1)above-and belowground biomass of R. soongorica were increased significantly with elevated CO₂, and this effect was promoted or inhibited when precipitation increased or decreased, respectively; When the CO₂ concentration was increased from 350 to 700μmol/mol, the aboveground biomass increased by 61.28%with precipitation plus 30%(P < 0.05), whereas root biomass was increased by an average 84%with precipitation 30%,and 3.21%with precipitation-30%, respectively (P < 0.05); Therefore, the root/shoot ratio(R/S) of R. soongorica with precipitation plus was greater than that with precipitation minus, and this effect was significantly inhibited by elevated CO₂(P < 0.05).(2) Elevated CO₂ significantly increased the carbon content in the root, stem, and leaf of R. soongorica, and significantly decreased nitrogen content in those organs; this effect was promoted or inhibited with precipitation plus or minus. The C/N ratio of the root, stem, and leaf of R. soongorica increased by 80.22% (root), 103.02% (stem) and 199.88% (leaf) with precipitation 30% (P < 0.05), whereas, the C/N ratio of those organs increased by 24.99% (root), 30.27% (stem), and 104.45% (leaf) (P < 0.05) with precipitation -30%, and elevated CO₂ significantly promoted the effect (P < 0.05).(3)These results suggested that, under future conditions of elevated CO₂, R. soongorica would be restored due to sufficient carbon and water resources in the area where precipitationis increased; Whereas, in areas where precipitation is decreased, R. soongorica would retain its dominant position for the high root/shoot ratio, because there would be compensation of elevated CO₂ to drought stress.