Global warming affects species diversity and productivity and their relationships. Lots of studies on relationships between species diversity and productivity have been done at the global and regional scales, but few studies have been done in the alpine grassland of Tibetan Plateau under the background of climate warming. This study explored the relationship between species diversity and productivity and its influencing mechanism by using measured data from 35 sample sites in alpine grasslands, combined with regression analysis, Pearson correlation analysis, variance partitioning analysis and structural equation model. The results showed that: (1) The species richness index and the Shannon-Wiener index of alpine grassland were significantly positively correlated with the net primary productivity of aboveground vegetation (ANPP), and the ANPP was more sensitive to the change of the species richness index. (2) The species richness index was positively correlated with longitude, soil available nitrogen, soil available phosphorus and mean annual precipitation. The Shannon-Wiener index was negatively correlated with altitude and latitude, but positively correlated with mean annual temperature. The ANPP was positively correlated with longitude, soil available nitrogen, mean annual precipitation and mean annual temperature, but negatively correlated with altitude, latitude and soil available potassium. (3) The interaction of geographic factors, soil nutrient and climatic factors had the largest contribution on the species richness index and ANPP with 10.99% and 32.91%, respectively, while the interaction of geographic factors and climatic factors had the largest contribution on the Shannon-Wiener index with 13.61%. (4) Geographic factors indirectly affected species diversity and ANPP through the regulation of soil nutrient and climatic factors while soil nutrient and climatic factors directly affected species diversity and ANPP. The results reveal the comprehensive regulation mechanism of environmental factors on species diversity and productivity, and provide a basis for the scientific response to climate change in the alpine grassland ecosystem of Tibet.