Algal blooms caused by excessive growth and reproduction of phytoplankton are becoming frequently in lakes, threatening their health and safety of ecological systems. In recent years, the coverage area of cyanobacterial blooms in Lake Hulun has been expanding. In 2022, the most severe cyanobacterial blooms almost covered the surface of Lake Hulun. A sampling program was conducted in the spring, summer and autumn of 2022, concerning the variables of phytoplankton species, cell density, biomass and several water quality indicators in the surface, middle and bottom water of the lake. A total of 193 species of phytoplankton were identified belonging to 113 genera and 8 phyla, with 14 dominant species. The species composition was of the green algae-diatom-cyanobacteria type. The number of phytoplankton species in spring and autumn was basically the same, that was twice of the minimum level in summer. The cell density in summer was the highest, which was one order of magnitude higher than that in spring and autumn. The phytoplankton biomass in spring, summer and autumn was 686, 510 and 166 mg/L, respectively. The Shannon-Wiener diversity index was the highest in spring. The Margalef richness index was the highest in summer. The Pielou evenness index was the highest in spring. The characteristics and seasonal differences of phytoplankton community were mainly caused by lake water environment conditions and meteorological factors. The correlation analysis revealed that the main influencing factors were water temperature, total nitrogen, total phosphorus, pH, dissolved oxygen, air temperature, evaporation and wind speed.