In order to reveal the effects of competition of other algae and Microcystis on growth of Microcystis under flow condition before the formation of algal blooms, and the growth of Microcystis aeruginosa is affected by Scenedesmus quadricauda under flow conditions, experiments are conducted by plexiglass ring groove in axenic laboratory. The competition relationship affected by competition parameters, density and specific growth rate of Microcystis are analyzed. The results show that the Microcystis aeruginosa adaptability on flow water is reduced under the competition between Scenedesmus quadricauda and Microcystis aeruginosa, namely, the optimal flow rate in simple and mix culture are 35 cm/s and 5 cm/s respectively. The exponential growth time of Microcystis aeruginosa under mix culture are longer than under simple culture. Microcystis aeruginosa growth are stimulated and its specific growth rate increase by Scenedesmus quadricauda at flow rate from 0 to 25 cm/s, contrary to these results at 35 cm/s. The growth of Scenedesmus quadricauda is slightly inhibited by Microcystis aeruginosa in control group and treatment groups during the experiment.