Eutrophication of urban lakes is a growing threat. Previous studies on algal blooms have focused on large natural freshwater reservoirs, but the dynamics of algal blooms in small urban shallow lakes are largely unknown. To identify the key factors affecting algal blooms in urban lakes, this paper analyzed the characteristics of phytoplankton variation and the relationship between phytoplankton and contributing factors in a typical urban lake, Moon Lake in Ningbo, China. During the bloom, the nutrient level of Moon Lake was between moderately eutrophication and extreme eutrophication. A total of 61 genera of phytoplankton belonging to 8 phyla were found and were dominated by Chlorophyta (51.79%) and Bacillariophyta (21.43%). The results showed that water temperature and light were found to be the main factors for phytoplankton growth, which was characterized by a succession of Cryptophyta, Bacillariophyta, Chlorophyta and Cyanophyta. Chlamydomonas reinhardtii caused the bloom and had a population density as high as 1.55×10⁸ cells/L. As algae density increased, the proportion of Chlamydomonas spp. increased (reached 81.10%), while the species numbers and community stability of the phytoplankton decreased. The subsequent Pearson correlation analysis and redundancy analysis also demonstrated that the growth of Chlamydomonas was positively correlated with water temperature, pH and total phosphorus. Rising temperature in the spring season, continuous sunny weather, high nutrient loads in urban shallow lakes and poor fluidity of water body helped create suitable living conditions for Chlamydomonas spp. with flagellum. Under these circumstances, the Chlamydomonas spp. with the greatest growth potential outnumbered other algae species in the competition of nutrients, light and other resources, resulting in the green algae blooms.