Abstract: Taking Changtan Reservoir as the research object, the characteristics of water environmental factors and phytoplankton community were analyzed, and the spatial-temporal distribution characteristics of nitrogen and phosphorus nutrients in water body were discussed. The effects of land use type variation on water quality and the main environmental factors driving the seasonal change of phytoplankton community were clarified. The corresponding suggestions were put forward aiming at the main environmental problems. The results indicated that the non-point source pollution of cultivated land and forest land was the main source of total phosphorus. The retention rates of TN and TP were 41.71% and 51.17%, respectively, in the reservoir impoundment area, resulting in the formation of N/P ratio of 34.5 in this reservoir. The phosphorus-restricted water body led to the advantage of cyanobacteria growth. The seasonal abundance of cyanobacteria ranged from 33.72% to 82.47%, and the dominant phytoplankton species were cyanobacteria. The toxigenicCylindrospermopsis raciborskii had the greatest dominance (Y=0.078). The redundancy analysis (RDA) results indicated that the environmental factors explained 61.1% of the succession of community structure. The water temperature, nitrogen and phosphorus nutrients concentrations were the main driving factors for community succession. The concentration of ammonia nitrogen ( ${\mathrm{NH}}_4^{+}$ -N) was significantly correlated with the abundance of Cylindrospermopsis raciborskii (correlation coefficient 0.999, P<0.02). The abundance ofCylindrospermopsis raciborskii could be reduced by 51% when the domestic sewage treatment rate was increased to 95% and the ${\mathrm{NH}}_4^{+}$ -N in effluent was lower than 1.5 mg/L. The main measures and countermeasures were put forward, including constructing non-point source pollution isolation zone, regulating and shortening hydraulic residence time in the reservoir area to reduce the growth advantage of cyanobacteria and improve the water ecological condition of Changtan Reservoir.