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To better understand how warming, increased precipitation and their interactions influence community structure and composition, a field experiment simulating hydrothermal interactions was conducted at an annual forb dominated desert steppe in northern China over 2 years. Increased precipitation increased species richness while warming significantly decreased species richness, and their effects were additive rather than interactive. Although interannual variations in weather conditions may have a major affect on plant community composition on short term experiments, warming and precipitation treatments affected individual species and functional group composition. Warming caused C-4 grasses such as Cleistogenes squarrosa to increase while increased precipitation caused the proportions of non-perennial C-3 plants like Artemisia capillaris to decrease and perennial C-4 plants to increase.