Small urban lakes are highly sensitive to changes in their environment and are closely associated with the surrounding urban systems. The distribution and succession pattern of phytoplankton can significantly influence the entire ecosystem in a small urban lake. From March to December 2015, phytoplankton and water quality were monitored in Sand Lake, Wuhan city, China. In total, 312 species were detected representing 10 phyla, with Chlorophyta, Bacillariophyta, and Cyanophyta being the major phyla represented. Biodiversity indices were calculated using the methods of Shannon-Wiener, Simpson, Pielou, and Whittaker. The biodiversity indices and environmental factors were subjected to correlation analysis, principal component analysis, and cluster analysis. Three interspecific association indices and the calibrated coefficients of χ2 test were calculated. The results showed that the phytoplankton reached a stable state in summer and autumn, during which the environmental factors were the main influence; the effect of constant ion concentration was greater than that of temperature, particularly those of sodium and magnesium. Analysis of the data for the dominant species showed that Chlorophyta and Cyanophyta were significantly correlated, whereas Bacillariophyta showed no significant correlation with other phyla. A significant correlation occurred between summer and autumn dominant species, while dominant species in winter and spring showed no significant correlation with those in other seasons, which could be attributed to the low stability of the phytoplankton during this period.