Carbon dioxide (CO_2) is the most important anthropogenic greenhouse gas. Global atmospheric CO_2 may reach 700 muL/L by 2100 and increase at a high rate of 0.4% per year according to the Intergovernmental Panel on Climate Change. Elevation of atmospheric CO_2 can have an indirect influence on global climate change through the greenhouse effect" and a direct influence on plant growth, survival, and reproduction. Therefore, elevated CO_2 will have a great effect upon structure and function of terrestrial ecosystems and on the distribution and productivity of global vegetation. Global warming has recently resulted in uneven precipitation and more frequent extreme droughts and shortages of available soil water in many areas of the world. In addition, plants have different sensitivities to elevated CO_2 and drought stress, even when growing in the same environment. Thus it is essential to consider both elevated CO_2 and different soil moisture conditions in order to assess the possible effect of global climate change on different species. In this paper, the six annual species, Rumex chalepensis, Vicia sepium,Sedum album, Hemmistepta lyrata, Clinopodium chinense and Chenopodium album, were treated with two levels of CO_2 concentrations (400 muL/L and 700 muL/L) and three levels of drought stress (CK: 100% FC (field capacity),MS: 40% FC and SS: 20% FC) in walk-in CO_2 chambers to determine the responses of growth and biomass allocation to the interaction of elevated CO_2 and drought stress and to test whether elevated atmospheric CO_2 ameliorates negative effects of drought by increasing water use efficiency. Results showed that V. sepium and S. album survived under all experimental conditions. However, there was some mortality in other species in drought treatments. Elevated CO_2 stimulated the growth of the five C3 plants R. chalepensis, V. sepium,H. lyrata, C. chinense and C. album under three levels of drought stress, while it inhibited the growth of S. album under drought stress. Drought stress inhibited the growth of all six species, but the growth of the CAM plant S. album was fastest under drought stress. The interaction of elevated CO_2 and drought stress showed significant interspecific variation: elevated CO_2 concentration alleviated the negative impact of drought on H. lyrata and C. chinense but less so in R. chalepensis and V. sepium; elevated CO_2 had no effect on the impact of drought on C. album; for S. album, elevated CO_2 even inhibited its growth under drought stress. Overall, elevated CO_2 increased the root mass fraction (RMF) and dry matter content (DMC) for most of the species; drought stress obviously increased RMF and decreased DMC in all six species. However, different species had different responses to the interaction of elevated CO_2 and drought stress. This indicated that plants could adapt to the interactive effect of elevated CO_2 and drought stress by regulating biomass allocation and moisture retention capacity, as dependent on the trade-off between carbon absorption and water loss. The results could help us to understand the adaptation of annual herbs to future climatic change and provide a basis for assessing and evaluating global climate change and its hydrological effect on plant physiological ecology.