Abstract: Elevation is one of the decisive factors of mountain plant species, population, community, and ecosystem. In this paper, an investigation was conducted on the changes of plant diversity and biomass in alpine grassland along an elevation gradient (2460-3260 m) on the southern slope of Tianshan Mountains. With the increase of elevation, environmental factors changed. The species richness index and Simpson dominance index appeared a unimodal pattern and peaked at the altitudes 3060 m and 2760 m, respectively, and the Shannon index and evenness index increased. The biomass also presented a unimodal pattern, and the above and belowground biomass peaked at altitudes 3160 m and 3060 m, respectively. Our results supported the “intermediate height expansion” theory. The belowground biomass was mainly concentrated in 0-10 cm soil layer, accounting for 63.4%-94.7% of the total belowground biomass, and the average root/shoot ratio was 62. Air temperature and soil water content were the main environmental factors affecting the species diversity. Besides water and heat factors, soil pH was the factor limiting the plant growth, whereas soil available P was the factor limiting the plant root growth. Correlation analysis indicated that species richness was very significantly correlated with aboveground biomass and significantly correlated with belowground biomass, Shannon index was significantly correlated with aboveground biomass, and species diversity had a linear correlation with total biomass.