In order to explore the temporal-spatial pattern of phytoplankton distribution in Hongze Lake and associated factors, we sampled surface water 20 times at 11 sites across the lake from July 2011 to October 2013, and obtained physical and chemical indices of phytoplankton and water quality. We processed the data using the Shannon-Wiener index H and dominance index Y. Then, we found 144 phytoplankton species belonging to 60 genera, and seven phyla(including 66 species of green algae, 36 species of diatoms, 23 species of cyanobacteria, and 13 species of euglena). The phytoplankton distribution presented an obvious spatial-temporal variability. The species richness was at a high level in the summer, at an intermediate level in the autumn, and at low levels in the winter and spring; it exhibited large values in the northwestern part(Chengzi Lake area)and the inflow area of the lake(site Lihewa), and small values in the central area and the outflow area of the lake. The average cell density of phytoplankton ranged from 1. 57×10⁶ to 6. 04×10⁶ cells/L. The cell density was higher in the summer and autumn, and lower in the winter and spring; it exhibited large values in the Chengzi Lake area and small values at the inflow and outflow areas of the lake. Data analysis shows that the temporal dynamics of the species composition and cell density were significantly associated with the seasonal variations of water temperature, water level, and nutrients(mainly total nitrogen and total phosphorus), while the spatial dynamics of the species composition and cell density were correlated with the spatial variations of hydrodynamic factors and nutrients(particularly ammonia nitrogen). We suggest restricting nutrient input and modulating the hydrological dynamics of Hongze Lake, in order to prevent algal blooms in the lake.