Abstract: In order to learn about the current situation of phytoplankton community structure and water quality in the Huanghe Reservoir, four samplings of the reservoir water were taken respectively in four seasons.The eutrophication status of the Huanghe reservoir was comprehensively evaluated by means of the single factor, trophic level index TLI (∑) , phytoplankton stock and species composition.The results showed that:in terms of the water quality, the major pollutant is nitrogen nutrient salt with its annual average value reaching up to 3.6 mg/L.TLI (∑) ranged from 37-59, with the highest value and the most serious eutrophication detected in winter.A total of 75 genera (falling into 6 phyla) of phytoplanktons (chiefly Cyanophyta and Chlorophytawere) were detected throughout the year, of which the dominant genera were found to be the Pseudanabaena, Raphidiopsis, Cylindrospermopsis, Oscillatoria, and Ankistrodesmus.The phytoplankton density varied in the range of 1.68 to 3.62×107 ind/L, and the biomass varied from 5.6 to 12.9 mg/L.Among the 6 phyla of phytoplanktons, the cyanobacteria was found to have the largest number (reaching >107 ind/L) throughout the year, indicating a risk of water algal bloom in the waters.At the same time, the dominant genus, Anabaena, has the potential to produce microcystic toxins, which may cause greater far-reaching damage to the waters.Juding from the indices of dominant phytoplanktons, community structure, density, biomass, and TLI (∑) , the reservoir water quality is in a state of medium to eutrophication.Thus without a better water quality management, a risk of cyanobacteria bloom might come true.