When the concentration of nitrogen and phosphorus reaches the eutrophication level, hydrodynamic conditions (flow, flow rate, stage) may be the constraints, which will restrain the development of algal bloom. Comparing the reservoirs of river-type and draining off water from deep layer with that of lake-type and draining off water from shallow layer, the former reservoirs have a low biomass of phytoplankton and algae, because of the large traffic exchange and low concentration of nutrient. After further study on hydrological conditions (residence time, stage change, water flow motion), the paper discovered that keeping short residence time, higher water level and lower water flow in bottom water layer is benefit to the dilution of concentration of nutrition and algae, is disbenenifit to the growth of phytoplankton and algae, witch slows down the eutrophication process.