This study analyzed the role of algae and bacteria in algal-bacterial symbiotic systems for the treatment of dyeing wastewater. The mechanism was investigated by constructing an algae-bacteria tandem system (A system) and a bacteria-algae tandem system (B system). The results showed that the chemical oxygen demand (COD) removal and decolorization rates of A system reached 91% and 90%, respectively, under optimal conditions, which were higher than that of B system. The degradation pathways of A and B systems differed according the degradation product analysis. High-throughput sequencing analysis revealed that Proteobacteria were the dominant bacteria in A and B systems. Armatimonadetes increased considerably in A system. These results show that algae mainly contributed to the preliminary degradation of anthraquinone dye, and resulting products were easily degraded by bacteria. This study provides guidance on the optimization of the system. It can be better adapted to the actual needs of wastewater treatment plants.