Abstract: Composition of algae and longitudinal variation in Cyanobacteria abundance and richness are distinct in the river downstream of a eutrophic lake, since the river receives a large amount of lacustrine phytoplankton from the lake. This paper examines the compositions of Cyanobacteria in the Tanglang River located downstream of the Dianchi Lake, and analyzes the driving factors of spatial variations in the abundance and richness of Cyanobacteria using the taxonomical method and statistical methods. The results show that the dominant algae in this river is Microcystis, a component that contributes 79.77% - 97.09 % of the abundance. And the abundance follows a law of exponential decay down the river stream and is featured with a significant inverse correlation with the length of river reach and hydraulic retention time (HRT) and no obvious correlations with water temperature (T), velocity, TP or TN. However, the richness is highly correlated with T (P < 0.01, R = 0.78**) and inversely with velocity (P < 0.05, R = -0.70*). Correlation analysis also indicates that the turbulent hydraulic condition is responsible for the longitudinal variation in Cyanobacteria abundance along the river while T and velocity have significant impacts on Cyanobacteria richness.