Abstract Algal–bacterial symbiotic system is ubiquitous in natural water and has an important impact on water quality and environment such as algal toxins from extracellular secretions. In general, the species diversity and abundance of this system can be dynamically regulated by quorum sensing (QS). However, very little is known about extracellular secretions of algal–bacterial symbiotic system under the QS process. In this study, the effects of N-acylhomoserine lactones (AHLs) on the algal–bacterial symbiotic system were investigated. When the supplemental concentration was between 250 and 500 μg/L, the biological dry weight, Chl-a, and phycocyanin of algae cells and the fluorescence intensity of extracellular humic acid and fulvic acid increased greatly. Whereas, when the supplemental concentration was greater than 750 μg/L, the concentration of Chl-a and phycocyanin showed opposite patterns. This result indicates that AHLs can mediate communication between algae cells and bacteria, and bacteria can regulate the growth of algae by secreting signal molecules. The results of this study also provide useful information for blue-green algae outbreaks prediction by changing trend of AHLs in water.