Microalgae are widely distributed in nature, which are easy to culture, grow fast and have high application value. They are widely used in biofuels, medical raw materials, high-quality food sources and animal husbandry. In recent years, the production of microalgae has been improved by the modification of photo-bioreactor, the screening of high yield algae and the transformation of metabolic pathway genes, and the research and innovation in the downstream process of microalgae treatment are not enough, especially the harvest of microalgae biomass has become the bottleneck of its industrial development. This paper summarizes the function of flocculation on the harvest of microalgal biomass, focus on the flocculation activity of flocculating microorganisms to harvest algal biomass, and the flocculation mechanism of flocculating microorganisms on microalgae is also discussed, Hopefully, it will provide some useful information and guidelines for the harvest of microalgal biomass by flocculating microorganisms.