There are water quality changes both the reservoir and downstream after hydropower projects construction.Analysis of these changes have important significant in assessing effects of hydropower projects on the aquatic ecosystem and riparian ecosystem.This paper investigates temporal-spatial characteristic of Lancangjiang River water quality along the middle reaches through river section monitoring and sample analyses selecting several representative indices including metals taking Cu,Zn,Pb and Cd for example,NH_3-N and P(total phosphorus and/or PO_4-P) in the past more than ten years.Results showed that after dams built spatial heterogeneity of metals was more evident than before along the whole middle reaches and not evident along certain reservoir.Moreover,ATUs(Acute Toxic Units) that could assess the changes of metal factors were lower than before and became lower and lower along the middle reaches.Adversely,temporal distribution of metal factors besides NH_3-N and P were more homogeneous.NH_3-N and P concentrations in reservoirs were higher whose maximums were tested at site about several kilometers downstream from the dam.There were increasingly eutrophicating after dams built.It was concluded that hydropower projects construction dominantly effected the spatio-temporal distributions of water quality.