[Background] Cobetia marina can produce many extracellular products with flocculation activity and therefore is regarded as a novel bioflocculant. Desmodesmus sp. F51 is one of the best new resources for natural xanthophyll due to its ability to accumulate large amounts of lutein. However, up to date there is no report on the use of bioflocculant from Coberia marina for Desmodesmus sp. F51 harvesting. [Objective] The flocculation efficiency and mechanism of the new biological flocculant on Desmodesmus sp. F51 were investigated. [Methods] Effects of bioflocculant addition in different growth periods, the addition amount of bioflocculant, flocculation time and pH on flocculation efficiency were investigated. The functional groups of bioflocculant were analyzed. Zeta potential of Desmodesmus sp. F51 before and after the addition of bioflocculant under different pH conditions were measured, and the morphology of Desmodesmus sp. F51 under the microscope before and after the addition of bioflocculant were also analyzed. [Results] The highest flocculating efficiency (82.1%, 15 min, pH 8.0) was achieved when 2 mL of bioflocculant was added at the stationary growth phase of Desmodesmus sp. F51. Fourier transform infrared spectroscopy analysis showed the characteristic structure of polysaccharides and amide. Thus, it was speculated that the bioflocculant was mainly a mixture of heteropolysaccharides, containing a small amount of proteins. According to Bradford method, the protein content in the bioflocculant was about 0.4% (w/w) and the total sugar content determined by phenol-sulfuric acid method was about 34.5% (w/w), which was basically consistent with the results of fourier transform infrared spectroscopy analysis. The flocculating efficiency is above 60% when the pH value ranges from 4.0 to 11.0, which indicates that the flocculation efficiency is higher under both acidic and alkaline conditions. The analysis of Zeta potential indicates that the dominant flocculation mechanism of the bioflocculant may be adsorption bridging. [Conclusion] This study is of great significance for microalgae harvesting by bioflocculation.