Increases in atmospheric nitrogen deposition and changes in precipitation patterns are important phenomena related to changes in the global environment. These changes have created a series of increasingly serious ecological problems affecting the structure and function of grassland ecosystems. Therefore, grasslands have become a focus of ecological research in China and around the world in recent years. High nitrogen deposition can acidify grassland soils and lead to nutritional imbalances, decreases in biodiversity and productivity, and grassland degradation, all of which are serious threats to the function of grassland ecosystems. Precipitation can increase the efficiency of nitrogen fertilizer used by plants, and climate warming is accompanied by changes in precipitation. Studies of the interactive effects of nitrogen deposition and variation in rainfall on natural grassland ecosystems can help to increase our understanding of the responses of such ecosystems to changes in the global climate. In China, the Stipa baicalensis steppe, a type of meadow steppe, is mainly distributed in the forest steppe zone of the Songliao Plain and the east Inner Mongolian Plateau. In Inner Mongolia,farmers mainly use S. baicalensis steppe as natural pasture; therefore, it has an important role in livestock production. The aim of this study was to evaluate the responses of grassland plant community species diversity and the aboveground biomass of several common plant species to nitrogen deposition and irrigation. We set up a factorial experiment to test the interactive effects of eight nitrogen treatments and water addition in S. baicalensis steppe in Inner Mongolia in June 2010. This involved nitrogen (NH_4NO_3) application levels of 0 (the control treatment, CK), 15, 30, 50, 100, 150, 200,300 kg N hm~(-2) a~(-1) (referred to as NO, N15, N30, N50, N100, N150, N200,N300, respectively) and simulated 100 mm summer rainfall. In mid-August 2013, we established 96 1 m * 1 m sample plots. The vegetation was harvested from each plot and the biomass of each species was measured to determine the effects of nitrogen and water addition on species diversity and the aboveground biomass of several common plants. Nitrogen and water application reduced plant species diversity in the steppe community, and there was a significant interaction effect between nitrogen and moisture. When water was added, increasing nitrogen application levels resulted in decreases in plant species richness, Shannon-Wiener index, and Pielou evenness index. When water was not added,increasing nitrogen application levels resulted in changes in species richness,Shannon-Wiener index, and Pielou evenness index that showed a "single-peak",but overall downward trend. Plant species differed in their responses to nitrogen and water addition. With increasing nitrogen application levels,the aboveground biomass of Leymus chinensis increased significantly, reaching a maximum at N300; the aboveground biomass of S. baicalensis, Achnatherum sibiricum,Cleistogenes squarrosa, Carex duriuscula, and Artemisia frigida first increased and then decreased,showing a " single-peak " trend; and the aboveground biomass of Potentilla acaulis,Thermopsis lanceolata,Melilotoides ruthenica,and Filifolium sibiricum decreased gradually. There was a significant interaction effect between water and nitrogen application level on the biomass of S. baicalensis, A. sibiricum, and M. ruthenica. We concluded that the changes in plant species diversity and aboveground biomass were related to nutrient application levels, water availability, their own characteristics, and interspecific and intraspecific competition.