Experimental studies on the interaction of submerged plant and phytoplankton in an eutrophic lake revealed complex mutual inhibition competition. Keen competition for the light in the water was found and phytoplankton took remarkable dominance. Submerged plant was the stronger competitor for mineral nutrients because it could get nutrients from both the lake water and the fertile sediment at the bottom. When light and nutrients were abundant, phytoplankton was suppressed by submerged plant with some unknown biochemical which caused falling of phytoplankton. The competition equilibrium was obviously affected by water depth, water temperature and nutrient concentration in the water. Phytoplankton would prefer high temperature, deep water and high nutrient level, bat submerged plant the opposite. Once the community of submerged plant grew up, it gave stronger suppression to the phytoplankton and kept its dominance more stable. Thus, submerged plant should be planted in the cool and low water level season in eutrophic waters with the external nutrient source strictly controlled.