Vegetation is an important part of wetland ecosystem in Poyang Lake National Nature Reserve and the analysis of its spatial-temporal variations and the influences of water level changes offer a better understanding of biotic community and habitats of immigrating birds.Hence,the primary concern of this paper is the effects of varying water levels on spatial-temporal variations of biomass in Poyang Lake National Nature Reserve based on MODIS EVI time series data.We also made use of biomass measurements obtained from a field campaign and hydro-meteorological data to assist our study.The main results are as follows: (1) Multi-year annual mean biomass density is between 0 and 1402 g/m².Except Lake Banghu which shows a radial distribution with lower biomass in the center and higher biomass outside,biomass distribution for the rest of the area generally decreases from north to south and is closely related with elevation.(2) Biomass density for flood recession period is about 901 g/m² and firstly increases and then decreases with time.The size of the area of vegetation community increases with the exposure of lake bottom and thereafter remains unchanged.The total biomass exhibits a unimodal distribution with a maximum in November.The annual mean values of biomass from 2001 to 2010 bear comparative fluctuations with the ten-year mean value of 18.3×10⁷ kg,and maximum of 28.2×10⁷ kg in 2006,and minimum of 8.37×10⁷ kg in 2010.(3) Annual mean biomass is significantly related with the exposure days of lake bottom and with the size of the area of vegetation community,with a correlation coefficient of 0.719 and 0.865,respectively.To sum up,the water level is a key factor of biomass variations.