<p>Astaxanthin is a naturally derived antioxidant with great market value. A huge biomass of cyanobacteria is harvested during water blooms salvage and treatment, of which <i>Microcystis aeruginosa</i> is a typical species with astaxanthin biosynthetic capacity. To promote the high-value utilization of the salvaged cyanobacteria after enhancing their astaxanthin accumulation via H₂O₂ treatment, the cellular proliferation, reactive oxygen species (ROS) content, photosynthetic performance, astaxanthin biosynthesis, as well as related gene expression in <i>M. aeruginosa</i> were assayed under H₂O₂ stress. With raising H₂O₂ concentration, the inhibition on cellular proliferation enhanced gradually, because of the increase of ROS levels and decrease of chlorophyll content and photosynthetic performance. H₂O₂ treatment increased the levels of astaxanthin along with its precursors, zeaxanthin and β-carotene, by up-regulating 7 genes (<i>crtP</i>, <i>Z-ISO</i>, <i>crtQ</i>, <i>crtH</i>, <i>cruA</i>, <i>cruP</i>, and <i>crtR</i>) involved in astaxanthin biosynthesis, with the maximum accumulation was observed after 2 days. For astaxanthin yield, it showed the strongest correlation with H₂O₂ concentration and astaxanthin content. Therefore, H₂O₂ treatment is suitable for improving the high-value antioxidant levels, and a short-term treatment can induce its rapid accumulation.</p>