With the continuous input of exogenous sulfate (SO₄^2-), the SO₄^2- concentration in eutrophic lakes continues to increase. Long-term field monitoring results show that the SO₄^2- concentration in Lake Taihu gradually increases to a level of 96 mg/L in recent decades. In addition, the decay of cyanobacteria blooms in eutrophic lakes will increase methane (CH₄) production and emission. However, there is still a lack of relevant studies on whether the increase of SO₄^2- concentration in lake water will affect the CH₄ production process in sediments. In this study, a cyanobacteria-water-sediment microcosm system was constructed, and five treatments of sulfates (30 mg/L, 60 mg/L, 90 mg/L, 120 mg/L and 150 mg/L) were added to explore the dynamics of SO₄^2-, reducing sulfides (∑S^2-) and CH₄ in the water during the decay process of cyanobacteria under different SO₄^2- concentrations. The results showed that the sulfate reduction effect was the strongest on the 6th to 9th days when cyanobacteria accumulated and decayed, meanwhile, the SO₄^2- concentration in the water rapidly decreased to the lowest values of 7.65 mg/L, 8.87 mg/L, 21.21 mg/L, 41.14 mg/L and 56.54 mg/L, respectively. Coupled with the process of sulfate reduction, the concentration of ∑S^2- in the water column continuously increased to the highest values of 4.77 mg/L, 6.98 mg/L, 7.49 mg/L, 7.49 mg/L and 7.43 mg/L. From the 10th to 21st days of incubation, the concentration of SO₄^2- in the water maintained at a low level, and the concentration of ∑S^2- decreased gradually and approached 0. At the initial stage, CH₄ slowly increased. After SO₄^2- concentration declined, the CH₄ concentration gradually increased, and then rapidly rose from the 6th to 9th days. At the end of incubation, the accumulative CH₄ concentration decreased with the increase of initial concentration of SO₄^2- in water, which was 546.39 μmol/L, 207.24 μmol/L, 79.61 μmol/L, 37.25 μmol/L, and 5.56 μmol/L, respectively. There was an exponential negative correlation between CH₄ concentration and initial SO₄^2- concentration. Therefore, to accurately assess of methanogenesis process in eutrophic lakes, the influence of rising SO₄^2- concentration should be considered.