Using the modelling experiment, the responses of soil drought stress on A. ordosica, a dominant species of Maowusu sandland plant community, under high concentration of CO₂ carried in green house,is studied. Maowusu sandland is a typical one of arid and semi-arid sandland in China. Water is the most limited factor to A. ordosica growth and development. The results show that the different scales of soil drought stress made the growth and biomass of A. ordosica to be decreased, and the negative impact increased with the drought degree aggravated. The increase of atmospheric CO₂ concentration can be treated as a function of ″fertilization″ for growth of A. ordosica. Although, the impact of high concentration of CO₂ and soil drought stress on A. ordosica was negative, there existed still the ″fertilization function″ .Its physiological mechanism is that the CO₂ concentration increasing promoted the photosynthesis rate.