Abstract: [Objective] The project aims to determine the height,coverage,density and aboveground biomass of the dominant plant species of grassland communities in different months and calculate the important values and analyze the dynamic change of diversity indexes in the northern slope of Tianshan mountain.[Method] The meter ampere quadrates of different grassland types were set based on the crossing method,and community characteristic parameters were monitored from spring to autumn.[Result] There were some differences in the important values of different grassland types.The whole trend of important value from may to september showed decreasing trend;especially forage grass of Stipa capillata,Festuca ovina decreased obviously than the forbs important value.The plant community height,coverage and aboveground biomass showed high trend in late spring and early summer while low from August to September.The statistical results showed that the trend of diversity indexes was increasing first and then decreasing and then increasing,such as Simpson index D and Shannon-Wiener index H,evenness index Esw and richness index R,And Simpson index D and Shannon-Wiener index H and species richness R to aboveground biomass were significantly positively correlated,and evenness index to aboveground biomass was not significantly negatively correlated.[Conclusion] The preliminary study of the quantity characteristics of the plant communities in different grassland types showed a trend of increasing first and then decreasing with the seasonal variation and the species diversity increased firstly and then decreased.The results can provide data for supporting the rational utilization of grassland.