We have investigated seasonally the constitution,abundance and diversity of phytoplankton in Niyang River,the branch of the Yarlung Zangbo River,Tibet from 2008 to 2009,and taken quantitative analyses to study spatio-temporal characteristics of phytoplankton and their responses to environment using multivariate statistics method.The results are as follows.Firstly,phyto-plankton community in Niyang River belongs to 7 phyla,29 families and 48 genus,among them,Bacillariophyta is the dominant phytoplankton.Secondly,Shannon-Wiener diversity index for Niyang River is suitable to the theory about mid-altitude bulge, which is Shannon-Wiener diversity index (or Pielou evenness index) higher at the middle of Niyang River (or lower reaches of Niyang River),descending at either side of Niyang River.Thirdly,using Duncan comparison method to judge the difference among sampling sites and seasons for species richness,total abundance,Shannon-Wiener diversity index and Pielou evenness index,there is no significant difference among four sampling sites for four indices mentioned above.However,there is significant difference between the summer and other three seasons for species richness and total abundance.There is also significant difference between the winter and spring for Shannon-Wiener diversity index,and between the winter and other three seasons for Pielou evenness index.Seasonal succession of phytoplankton in Niyang River is mainly defendant on the supplement of exogenous water source,succession along Niyang River have a close relation with substrate of riverbed.Fourthly,based on the CCA method,there is a close relationship between water physical-chemical property and Naviculaceae algae from Bacillariophyta,and between water physical-chemical property and partial algae from Cyanophyta and Chlorophyta.Finally,we predicted the relationships between the total abundance of phytoplankton and environment factors based on CART method,the relationships between Shannon-Wiener diversity index of phytoplankton and environment factors based on CART method,and the relationships between Pielou evenness index of phytoplankton and environment factors based on CART method.The results reveal that pH value has a strong influence on the total abundance and Pielou evenness index of phytoplankton community in Niyang River.Pielou evenness index of phytoplankton is high at Niyang River sections where pH values are below 8 than those sections where pH values exceed 8.Shannon-Wiener diversity index of phytoplankton community in Niyang River is mainly influenced by substrate of Niyang River,specifically,Shannon-Wiener diversity index for the areas where the substrate is sandstone is higher than those areas where the substrate is clay.The paper also recommends paying close attention to phytoplankton and the environment factors mentioned above owing to their important indication function,in order to warrantee the sustainable development of Niyang River aquatic ecology.