In order to study the toxic effects of tetracycline on algae, Selenastrum capricornutum is selected as the test organism. The growing situation, chlorophyll a contents, superoxide dismutase (SOD) activity, malondialdehyde (MDA) content and reactive oxygen species (ROS) level in the cells of Selenastrum capricornutum after 0, 02, 04, 08, 16, 32 mg·L^-1 tetracycline exposured are studied by indoor test method. The results show that the tetracycline can strongly inhibit growth of Selenastrum capricornutum and the 50% effective concentration (EC₅₀) is 3.142 mg·L^-1 during 96 h exposured. Tetracycline treatment inhibits the content of chlorophyll a in the cells of Selenastrum capricornutum. During 96 h exposure period, with the increase of tetracycline concentrations, the activity of SOD shows a "Bell-shaped" trend, the content of MDA and the level of ROS shows an upward trend. The study indicates that tetracycline can cause oxidative stress and damage the antioxidant system in Selenastrum capricornutum cells. Tetracycline poses a potential risk to aquatic ecosystems.