Abstract: To understand the toxic effect and mechanism of perfluorooctane sulfonate potassium (PFOS) on antioxidant defense system of marine fish, we studied the changes of GSH content and GST activity in gill and liver of red sea bream (Pagrosomus major) during PFOS stress and water purifying under laboratory conditions. The results show that at the beginning of exposure, GSH content in gill was induced at low concentration but inhibited at middle and high concentration; GSH content was significant inducted(P < 0.01) with time, and on 15^th day of stress, GSH content had significant dose effect at various concentrations; GSH content was reduced with increasing PFOS exposure concentration. The change of GST activity was similar with GSH content in gill and liver. The biomarkers in some groups recovered to the level in the control during water purifying, which suggests red sea bream can repair damage caused by PFOS stress. Therefore, PFOS has obvious toxic effects on red sea bream under laboratory conditions, and may be harm to marine higher organisms. Risk assessment of marine environment should be concerned about.