Abstract: To investigate the effect of LED light on the photosynthetic activity of cyanobacteria, the photosynthetic activity of Microcystis aeruginosa PCC 7806 was examined after 2h of LED treatment with different light quality and intensity using 25 μmol photons/(m²·s) of white fluorescent light as control. When compared to the control, results indicated that the cell photosynthetic activity (F_v/F_m) soared significantly under 25—50 μmol photons/(m²·s) LED red and blue light, and 25—100 μmol photons/(m²·s) LED white and green light treatments. LED red and blue light significantly inhibited cell photosynthetic activity at 100 μmol photons/(m²·s), whereas this intensity was more than 200 μmol photons/(m²·s) for the LED white light and 500 μmol photons/(m²·s) for the LED green light, and the maximum photochemical efficiency of photosystem Ⅱ (PSⅡ) F_v/F_m, the electron transfer rate ETR (Ⅱ) and light quantum yield Y (Ⅱ); the photosystem Ⅰ (PSⅠ) electron transfer rate ETR(Ⅰ), and the light quantum yield Y(Ⅰ) decreased with increasing light intensity; with increasing light intensity, the degree of electron transfer blockage between \textQ_\textA^- and Q_B, the proportion of PSⅡ inactive reaction centers (PSⅡx) and the energy flux absorbed by a single active reaction center (ABS/RC), the total energy dissipated by a single active reaction center (DI₀/RC), the energy absorbed by the cell per unit area of algae (ABS/CS₀), and the energy dissipated by heat per unit area of algae (DI₀/CS₀) were increased. Although the flux of electrons transferred by a single active reaction center (ET₀/RC), the flux of energy captured per unit area of algal active reaction center (TR₀/CS₀), and the flux of energy transferred per unit area of algal electrons (ET₀/CS₀) were all decreased under the action of increasing light intensity (P<0.05), the flux of excitation energy captured by a single active reaction center (TR₀/RC) was not changed significantly (P>0.05). Under the action of different light qualities of the identical light intensity, the decline degree of F_v/F_m, ETR (Ⅱ), Y (Ⅱ), ETR (Ⅰ), Y (Ⅰ), the degree of electron transfer blockage between \textQ_\textA^- and Q_B, the increased degree of PSⅡx, ABS/RC, DI₀/RC, ABS/CS₀, DI₀/CS₀, and the decline degree of ET₀/RC, TR₀/CS₀, ET₀/CS₀ manifested most when subjected to red light therapy and least when subjected to green light treatment. These study results demonstrated that at low light intensities, the four LED light qualities significantly increased the photosynthetic activity of Microcystis aeruginosa and ensured the balance of energy distribution; high light intensities decreased the photosynthetic activity of Microcystis aeruginosa by affecting the PSⅡ reaction center and inhibiting electron transfer on the receptor side of the PSⅡ electron transfer chain, while the large amount of light energy absorbed by the cells protects and alleviates the stress of intense light stress by increasing heat dissipation and reducing the quantum yield of electron transfer. The mechanism of photosynthetic activity of Microcystis aeruginosa in response to LED light was revealed in this study, and it was confirmed that high light intensity LED light can suppress photosynthetic activity of algal cells, which is a feasible technical way to prevent and control the explosive proliferation of Microcystis aeruginosa.