Although lot of the studies on the effects of single factor, such as atmospheric CO₂ concentration and precipitation, on plant growth characteristics have been done, the effects of interactive CO₂ concentration and precipitation on plant growth characteristics have not been studied yet. Those restricted the comprehensive understanding and accurate prediction of the effects of global change on plant growth characteristics. Stipa breviflora is one of the dominant plant species in desert steppe and a kind of forage grass with high quality. Investigating the sensitivity and adaptive capacity of S. breviflora to climate change is significant for maintaining the stabilization of desert ecosystem and the security of stockbreeding. We had studied the effects of increased CO₂ concentration and varied precipitation as well as their interactions on the growth characteristics (such as plant height, leaf area, survival rate, leaf number and biomass, etc.) of S. breviflora based on the simulation experiment using open-top chambers (OTC) from June to August (the main growing season) in 2011. In this paper, three levels of CO₂ concentrations (control, 450 and 550 μmol/mol), five levels of precipitation (-30%, -15%, control,+15%, and+30% based on the average monthly precipitation from 1978 to 2007 in Siziwangqi county, Inner Mongolia) and their interactions were studied. There were six replicates (i.e., six pots with four plants per pot) for every CO₂ concentration treatment and precipitation treatment. The seeds were sowed on April 18^th and well watered before control experiment. Then 90 pots of plants with consistent growing vigor were randomly selected and placed into different chambers as different treatments. High-purity CO₂ was pumped into those chambers with high CO₂ concentration day and night, regulated by a CO₂ automatic control system. Each precipitation regime was converted to irrigation amounts of every month and then divided into 10 times to water, i.e. watered every three days after 4:00pm. The chambers were covered in rainy days to avoid extra water input. The whole experimental site was covered with sun-shading net to lower the temperature in chambers at 9:00-16:00 on clear days. Plant heights were measured in late July and late August, respectively. Leaf number, leaf area and biomass were measured at the stage with the biggest biomass. The results indicated that, comparing with the control, the growth characteristics of S. breviflora did not change significantly under 450μmol/mol CO₂ concentration. However, the biomass, plant heights (in July), leaf area and single leaf area of S. breviflora increased significantly under 550μmol/mol CO₂ concentration, while leaf number and plant heights (in August) did not change significantly. The changes of precipitation obviously affected the biomass, leaf area, leave number, plant height and survival rate of S. breviflora. The interactions of increased CO₂ concentration and varied precipitation had significant effects on the biomass of S. breviflora. CO₂ fertilization effect could offset the adverse impact induced by decreasing precipitation, but the effect of increasing CO₂ concentration would be reduced markedly when the precipitation decreased about 30%. This result would help to understand the adaptation of desert steppe to future climatic change and to make countermeasures to cope with climatic change.