Productivity and species diversity are important indicators of the service functions and basic factors of stability in grassland ecosystems. Precipitation is a main limiting factor of grassland productivity. There is a positive correlation between precipitation and productivity. Species diversity increases under precipitation enhancement; the water availability of the plant community has an important effect on community structure, composition, and species diversity. An increase in soil moisture content not only affects the grassland plant communities directly but also improves soil nutrient availability to promote plant growth. However, drought could lead to a series of negative effects on grassland ecosystems. Therefore, moisture is an important factor for productivity and species diversity, and it even affects the stability and sustainability of grassland ecosystems.Northern Tibet has low ecosystem stability, so external disturbances, such as climate change and human activities, easily result in variation in the pattern, process, and function of ecosystems. In recent years, Northern Tibet has experienced warming and increased precipitation. Increased precipitation will affect grassland productivity and species diversity, and will even influence animal husbandry. As a main measure of the adaptation strategy for climate change, irrigation could be a reasonable pathway to redistribute and make full use of increased precipitation.In this study, we conducted a growing season irrigation experiment in Northern Tibet (between 2008 and 2010) to simulate the precipitation enhancement in this area. We designed 4 irrigation gradients: CK (no irrigation), DS (low irrigation, plus 5% more than the ambient precipitation), ZS (moderate irrigation, plus 10% more than the ambient precipitation), and GS (high irrigation, plus 15% more than the ambient precipitation). We measured plant biomass, community composition, and species diversity under different amounts of irrigation. The results showed that irrigation had greater effects on alpine grassland productivity and species diversity in drought years than in wet years. Under the irrigation conditions, the grassland biomass increased significantly, and the highest increase was up to 116% in a high irrigation plot. Irrigation promoted the species importance values of alpine grasslands; in particular, the proportion of shrubs and broad-leaved forbs was increased. The proportion of dominant species decreased significantly under irrigation conditions, and the decrease was the highest in the high irrigation plot. The Simpson index and Shannon-Wiener index significantly increased under different amounts of irrigation (P < 0.05). The E. Pielou evenness index was not significantly influenced by irrigation (P > 0.05). There was a positive correlation between biomass and the Shannon-Wiener index (P < 0.05). In short, we can project that grassland biomass and species diversity will increase under future precipitation enhancement; this will reduce the negative impact of drought in alpine grasslands and promote the sustainable development of animal husbandry in alpine grasslands.