It is a severe environmental problem of algal blooms caused by eutrophication in lakes and reservoirs. Nitrogen is a limiting biogenic element for primary productivity. It is important for recognizing the role of nitrogen in the algal bloom. In this study, typical reservoir (Jiaodong Peninsula, China) with low carbon and high nitrogen was inoculated for simulation. The mechanism of how nitrates impacted on the compositions phytoplankton and microbial communities was explored by adding different concentrations of nitrate. Results displayed that (1) when NO^-₃-N was the only nitrogen source, the concentration of NO^-₃-N would decrease along culturing. The concentrations of NO^-₂-N and ammonia would gradually increase. It indicated that NO^-₃-N could be converted into NO^-₂-N and ammonia by algae and bacteria. (2) The chlorophyll-a reached to the highest concentration among all treatments while 6 mg/L NO^-₃-N was added. When the concentration of NO^-₃-N was higher than 6 mg/L, the concentration of chlorophyll-a decreased. (3) After adding NO^-₃-N, cyanophyta and chlorophyta became dominant algae in the algal bloom of simulation, and the relative abundance of proteobacteria increased. The results would provide a theoretical basis and technical support for understanding and effective prevention of blue-green algal blooms in low carbon and high nitrogen reservoirs.