Extreme drought and its impact on plant species diversity is one of the important issues of climate change and its impact assessment. However, there are few studies on the relationship between climate change and plant community diversity and its function in desert-steppe. To address this knowledge gap, responses of the Stipa glareosa community species diversity, aboveground biomass and its carbon and nitrogen density to extreme drought was examined by an in situ experimental platform in desertgrassland region in northern China, for enhancing our understanding of the effects of extreme drought on desert-grassland community. The results indicated that extreme drought changed the Stipa glareosa community species composition and diversity. In august, the two extreme drought treatments of reducing 66% of ambient rainfall from May to August and reducing 100% of ambient rainfall in June and July decreased community Shannon-Wiener index and species richness. Compared with the CK, the standing aboveground biomass was decreased more than 50% by two extreme drought treatments (P<0.05). And litters biomass was decreased 61% by reducing 66% of ambient rainfall from May to August (P<0.05). The standing aboveground biomass carbon, nitrogen density in the two extreme drought treatments was significantly lower than these of CK. The litters biomass carbon, nitrogen density was significantly reduced by reducing 66% of ambient rainfall from May to August. Consequently, extreme drought affected vegetation carbon, nitrogen density through altering species composition and aboveground biomass, and ultimately weakened the carbon, nitrogen sink function of desert-grassland ecosystem.