Atmospheric CO_2 concentrations are predicted to increase from approximately 350mumol/mol today to over 700mumol/mol in the late 21th century. In the future,elevated CO_2 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 CO_2,the interactions between changing precipitation and CO_2 on desert plants have attracted little attention to date. A pot experiment was conducted to study the interaction of elevated CO_2 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 CO_2 concentrations (350 and 700mumol/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) aboveand belowground biomass of R. soongorica were increased significantly with elevated CO_2,and this effect was promoted or inhibited when precipitation increased or decreased,respectively; When the CO_2 concentration was increased from 350 to 700mumol/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 CO_2 (P<0.05).(2) Elevated CO_2 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 CO_2 significantly promoted the effect (P<0.05).(3) These results suggested that, under future conditions of elevated CO_2,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 CO_2 to drought stress.