Surface water-blooms (SWBs) is usually explained as a buoyant migration of blue-green algae to the water surface under the conditions of reduced turbulence. Wind-induced turbulent mixing can cause considerable heterogeneity in the vertical concentration distribution of algal cells. There exists a criterion below which cell buoyancy can overcome the turbulent mixing. Former researchers regarded a certain wind speed as this criterion, but this wind speed differs in value. In this research, we propose a new criterion, a nondimensional parameter – Entrainment number (En), which represents the ratio of the intrinsic velocity of algal cells relative to the characteristic velocity of wind-induced mixing. Observation in Taihu Lake demonstrates that this new criterion is more suitable for the description of hydrodynamic effect on the formation of surface water-blooms.