The spatial-temporal characteristics of chlorophyll-a concentration and different phytoplankton densities were analyzed in this paper, respectively, based on the monthly monitoring data in Lake Taihu from March 2012 to February 2013. The correlation and stepwise multiple regression equations between chlorophyll-a, some key environmental factors and different phytoplankton densities were also studied. The results showed that the annual average of chlorophyll-a concentration in Lake Taihu was 22.33 ± 37.65 mg/m³, ranging from 0.48 to 347.85 mg/m³. There was an obvious temporal and spatial variation of chlorophyll-a concentration and phytoplankton density. The highest average concentration of chlorophyll-a occurred in summer, while the lowest in spring; the average concentration of chlorophyll-a was much higher at the north and northwest of Lake Taihu than that at the east and south. Cyanophyta, Crytophyta, Bacillariophyta and Chlorophyta densities changed as peak type, reaching a maximum in October. Xanthophyta, Chrysophyta and Euglenophyta densities changed in a V-type trend, reaching a maximum in spring or winter. The maximum of different phytoplankton densities almost all appeared in northwest region. Significant correlations were found between chlorophyll-a concentration and pH, dissolved oxygen, water temperature, nitrite, phosphate, total organic carbon. lg(Y_{Chl. a}) is negative correlated with lg(X_TN), significantly positive with lg(X_TP) and significantly negative with lg(X_N/P), respectively. Significant correlations were found between chlorophyll-a concentration and Cyanophyta, Crytophyta, Euglenophyta and Pyrrophyta densities.