The species diversity and individual abundance of planktonic algae were investigated and preliminary studied of the West Lake (Hangzhou,China) from 2006 to 2007. The water quality of the West Lake was evaluated as well according to water and wastewater monitoring and analysis methods. This article combine biological and chemistry experimental data to make an overall appraisal and put forward the proposal to species diversity protection and the water quality continuable use. In this studies, 179 species of planktonic algae were identified, species numbers belonging to phyla Cyanophyta, Cryptophyta, Chrysophyta, Pyrrophyta, Xanthophyta, Euglenophyta, Bacillariophyta and Chlorophyta were 25, 1, 3, 3, 5, 23, 41 and 78 respectively. In addition, the characters of species composition and distribution as well as the relationship between them and the changes of the water quality were analyzed. The results showed that the composition and distribution of species and water quality changes matched in principle. The water quality of the West Lake (Hangzhou) has been improved significantly than that in the past, some planktonic algae indicating clean water gradually increase in species number.