The main effects of irradiances（20,40,80,120,200 μmol/m2•s） on growth and biochemical composition of Sargassum muticum were studied indoors.Higher growth rates of S. muticum were observed at irradiance of 80~200 μmol/m2•s, and the maximum content of some biochemicals (chlorophyll-a, chlorophyll-b, soluble protein, and soluble sugar) were obtained at the lowest irradiance (20 μmol/m2•s). The rapid increase of algae polyphenols was observed at the highest irradiance (200 μmol/m2•s).The finding has important implication for the extraction of natural polyphenols.