Abstract: M. aeruginosa and S. quadricauda are two famous algae in Lake Taihu. The dominance of Microcystis is the premise for the formation of Microcystis blooms. At different phosphorus mass concentrations of 0.012, 0.112, 0.412 and 1.612 mg/L in the artificial Taihu lake water, S. quadricauda was all the superior competitor and the pH was higher than 10 at the stationary phase. When pH decreased to 5.8 using HCl (0.04 mol/L), both algae resumed to grow and S. quadricauda dominated at low phosphorus mass concentrations (0.012～0.412 mg/L) while M. aeruginosa dominated at high phosphorus mass concentration (1.612 mg/L). It suggested that the influence of pH on the competition of blue-green algae and green algae was interacted with phosphorus mass concentration.