Carbon dioxide (CO₂) is the most important anthropogenic greenhouse gas. Global atmospheric CO₂ may reach 700 μL/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 CO₂ 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 CO₂ 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 CO₂ and drought stress, even when growing in the same environment. Thus it is essential to consider both elevated CO₂ 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 CO₂ concentrations (400 μL/L and 700 μL/L) and three levels of drought stress (CK: 100% FC (field capacity), MS: 40% FC and SS: 20% FC) in walk-in CO₂ chambers to determine the responses of growth and biomass allocation to the interaction of elevated CO₂ and drought stress and to test whether elevated atmospheric CO₂ 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 CO₂ 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 CO₂ and drought stress showed significant interspecific variation: elevated CO₂ concentration alleviated the negative impact of drought on H. lyrata and C. chinense but less so in R. chalepensis and V. sepium; elevated CO₂ had no effect on the impact of drought on C. album; for S. album, elevated CO₂ even inhibited its growth under drought stress. Overall, elevated CO₂ 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 CO₂ and drought stress. This indicated that plants could adapt to the interactive effect of elevated CO₂ 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.