To evaluate the water quality and algal bloom occurrence in the eutrophic Wenruitang River, the eco-populations and photosynthetic activities of phytoplankton community in its surface water were investigated by high-frequency sampling during the day time in the middle of each month in 2015. The relationships between chlorophyll-a concentrations (Chl.a) and environmental factors such as water temperatures and nutrients were analyzed. It was found that the seasonal Chl.a were 39.98, 37.62, 21.59 and 10.74 μg/L with corresponding water temperatures 30.91, 25.34, 20.72 and 13.80℃ from spring to winter, respectively. The annual concentrations of total nitrogen (TN) ranged from 5.33±0.81 to 9.40±1.25 mg/L and those of total phosphorus(TP) ranged from 0.32±0.18 to 0.95±0.25 mg/L, belonged to a hypereutrophic level. The annual phytoplankton community in Wenruitang River was green-diatom algae type. Cyanobacteria were only found in the late spring and early summer and accounted for a small percentage. The relative abundances of cyanobacteria were 11.55%±7.99% and 12.70%±3.48% in May and June. Green algae were dominant in summer, but diatoms (along with a few dinoflagellates) were prevalent in winter. The relative abundances of the green algae increased with water temperature while the diatoms decreased when water temperature risen. Total Chl.a was in a postive correlation with water temperature, while no obvious correlations with TN and TP. The Chl.a showed distinct seasonal and diurnal patterns. However, the effective quantum yields (ΔF/F_m) of both green algae and diatoms decreased at first and then increased in a day, contrary to the diurnal changed in solar irradiances. The results indicated that the hypereutrophic state of Wenruitang River was potential for algal blooms but the possibility of cyanobacterial bloom was very low still due to its negligible relative abundance in a full year. In view of effects of temperature and solar radiation on Chl.a and physiological activities of phytoplankton in the surface water of such hypereutrophic resting waters, the sampling time and weather should be considered during field sampling.