Since the Three Gorges reservoir had been constructed and operated,the eutrophication problem in the Pengxi river has become increasingly prominent.The research was carried out in Spring and summer seasons when algal bloom is easy to take place from April to July,2014.The paper analyzed the structure feature of algal bloom diversity and situation of water quality in Pengxi river basin.The result showed that when alga bloom took place,the feature of water environment changed evidently and the situation of water quality is more serious.The density of algal cell is quickly increased and can got to 1176×104 cells/L. The species of algae is water bloom algae and pseudomonas aeruginosa.The algae species are single and the diversity of algae is significantly lower. The density of algal cell in downstream was much higher than that in upstream on space distribution.The relative analysis indicated that the density of algal cell is positively correlated with TN and TP.Algae proliferation outbreak has an important link with the increase of water nutrient concentration.The control of algae bloom in Pengxi River basin depends on effective governance of watershed nutrient salt emission.