Global climate change is predicted to alter rainfall patterns of the growing season, potentially reducing total quantities of growing season precipitation and redistributing rainfall into fewer but larger individual events. Such changes are likely to have profound effects on terrestrial ecosystems. Many soil, plant, and ecosystem properties in the arid and semi-arid regions of Northwest China could be affected, and this could ultimately affect species composition and biological diversity. Both the quantity of rainfall and the precipitation time interval influence seedling growth. Although many studies have examined the effects of a single environmental factor (e.g., temperature, precipitation, or carbon dioxide[CO₂]), or even two factors (e.g., precipitation and temperature or precipitation and CO₂), the interaction of changing precipitation and precipitation time intervals on plants, especially from multiple precipitation and precipitation interval regimes, have attracted little attention to date. Given the strong influence of rainfall on plant growth, it is important to understand the implications of these interactions.Reaumuria soongorica is a typical species of the arid regions of Northwest China. The responses of R. soongorica seedlings to altered precipitation patterns could reflect the general responses of this ecological system. Annual R. soongorica seedlings were used as a case study species to study growth traits, biomass accumulation, and biomass allocation under different precipitation patterns during the growing season. A ventilated rainout shelter was used for experimental manipulation of rainfall patterns to keep temperature and CO₂ close to natural conditions. Then we set up a controlled experiment with two variables:precipitation quantity and precipitation time interval. The three variations of precipitation quantity were -30% (W-); natural precipitation (W); and +30% (W+). The two precipitation time intervals were 5 days and 10 days. The results showed that:(1) with increased precipitation (+30%), the height and base diameter of seedlings increased by an average of 22.0% and 28.0%, respectively (P<0.05). However, these effects were even more significant when the precipitation interval was extended from 5 to 10 days; these conditions increased height and base diameter by an average of 24.57% and 32.98%, respectively (P<0.05). (2) Above ground, below ground, and total biomass were significantly increased by 241.57%, 223.95%, and 236.72% (P<0.05) when precipitation was increased by 30% and the precipitation interval was extended to 10 days, and the growth of above ground parts was marginally better than that of underground parts, but not obvious. (3) Compared to the control, the mean increase in root length was 21.0%, and the root/shoot ratio increased significantly by 53.73% (P<0.05). When precipitation was decreased by 30%, however, there were no significant changes in biomass accumulation and growth. Through this study we noted that both the quantity and time interval of precipitation significantly affected growth and biomass accumulation in R. soongorica seedlings; however, the effects of precipitation quantity were greater than those of the precipitation time interval. Nevertheless, the effects of precipitation still depended greatly on the time interval, with the strongest effects being observed following increases in precipitation with a concurrent extension of the precipitation time interval to 10 days. These results suggest that large events of increased rainfall and extended rainfall intervals in the future will be more conducive to biomass accumulation and growth inR. soongorica seedlings.