Abstract: In order to explore the influence of wastewater-borne bacteria of sludge dewatering liquid on the growth of Chlorella vulgaris and the interactions of algae-bacteria symbiotsis, three systems of co-cultivation of algae and bacteria（A+B）, cultivation of algae separately（A）, and cultivation of wastewater-borne bacteria separately（B） were set. The biomass of Chlorella vulgaris was 1.59 g/L in the system of co-cultivation, which was significantly higher than that in the system of cultivation of Chlorella vulgaris separately 1.24 g/L, after 72 hours of cultivation, indicating that the co-cultivation of algae-bacteria could promote the growth of Chlorella vulgaris. The monitoring of CO₂ and O₂ showed that Chlorella vulgaris and bacteria could utilize CO₂ and O₂ released each other, thus achieving mutualism. In the system of co-cultivation, the utilization rates of COD, TP, TN and NH⁺₄-N were 84.8%, 81.4%, 75.5% and 86.8%, respectively, which were all higher than that in the systems of cultivation of algae and bacteria separately. In addition, TP、 TN and NH⁺₄-N could be removed much more by the Chlorella vulgaris than that by the bacteria, and the utilization rate of COD by wastewater-borne bacteria and Chlorella vulgaris was similar. The results of the extracellular polymers showed that the protein and polysaccharide secreted by Chlorella vulgaris were significantly higher than that of wastewater-borne bacteria. The high-throughput sequencing results showed that four genera of Pseudomonas（Abundance was 44.2%）, Alicycliphilus（Abundance was 25.7%）, Gemmobacter（Abundance was 13.5%） and Flavobacterium（Abundance was 6.9%） were the dominant genara and could be symbiotic with Chlorella vulgaris in the co-cultivation system. Scanning electron microscope observed algae and bacteria growing together in clusters through extracellular polymer adhesion.