Abstract: Microcystis blooms are common in eutrophic water bodies in China. Some Microcystis species can produce microcystins (MC_S), which pose a serious threat to the safety of humans and ecosystems. The anionic surfactant linear alkylbenzene sulfonate (LAS), as a kind of widespread organic pollutant in eutrophic water bodies, its potential effects on the succession of toxic and non-toxic Microcystis and its influence on the toxicity of toxic Microcystis are often ignored. The effects of different LAS concentrations (0, 0.05, 0.2, 0.5, 1.0 and 5.0 mg/L) on the growth, photosynthetic characteristics, interspecific competition and toxin synthesis of toxic Microcystis aeruginosa and non-toxic Microcystis wesenbergii were studied in the present study. The results showed that the biomass, mcyD expression and MCs content of the toxic Microcystis increased significantly after 12 days of culture at 0.05—5.0 mg/L LAS. The concentrations of intracellular and extracellular MCs in toxic Microcystis were 0.069, 0.052, 0.061, 0.038, 0.037 fg/fg chl.a and 107.1, 103.7, 127.1, 99.6 and 113.7 ng/L, respectively. 0—0.2 mg/L LAS had no significant effect on the biomass of Microcystis wesenbergii, while a higher concentration of LAS (0.5—5.0 mg/L) significantly inhibited the growth of non-toxic Microcystis wesenbergii. When two Microcystis strains were co-cultured, the expression of mcyD in Microcystis aeruginosa responded quickly to LAS, the proportion of toxic Microcystis aeruginosa in the total Microcystis up to 60%—80%. But the synthesis and release of MCs were slow. A large number of MCs were synthesized and released after 12 days of co-culture. The intracellular MCs content was 0.098, 0.184, 0.201, 0.216, 0.168 and 0.241 fg/fg chl.a, respectively. In general, this research showed that LAS could promote the growth and toxin synthesis of toxic Microcystis. Therefore, the potential role of LAS pollution on the dominance formation of toxic Microcystis and toxin of blooms in eutrophic water can not be ignored.