Abstract: The semi-static toxicity test method was used to determine 96h acute toxicity of PFOS (perfluorooctanesulfonate) to Pagrosomus major so as to explore toxicity of PFOS to the fish and mechanism of the toxication and analyze effects of the substance varying in concentration, 0.1 mg•L^-1, 1.0 and 2.0 mg•L^-1 on SOD, CAT, POD activities and MDA content in gills and damage to the tissue . Results show that the 96h-LC₅₀ of PFOS to P. major was 22.56 mg•L^-1. In the group of fish exposed to 0.1 mg•L^-1 of PFOS,SOD activity in gills was induced first and then inhibited, while in the groups of fish exposed to 1.0 and 2.0 mg•L^-1 of PFOS,SOD activity was significantly inhibited (P< 0.05) and the SOD inhibition rate reached 14.86% on D15 in the group exposed to 2.0 mg•L^-1 of PFOS. CAT activities in all the groups followed a similar "U" pattern and the CAT activity induction rate reached 40.53% and 62.32% on D7 and D15, respectively, in the group exposed to 2.0 mg•L^-1 of PFOS. POD activity in the low dosage group increased significantly (P <0.05) on D1 and declined afterwards down to the level in the control, while in the moderate and high dosage groups, it was first inhibited and then induced, with induction rate reaching 63.05% on D15 in the high dosage group. The variation of MDA content did not show any pattern. At the end of clean water recovery period, SOD activity in all the three groups returned to the level in the control, while CAT and POD activities and MDA content still differed significantly between the PFOS treated groups and the control group (P<0.05). Under the stress of PFOS, the fish was found to have its gill filaments losing their epitheles and getting merged, and the lesion aggravated with the concentration of the toxicant and the duration of the exposure. The findings of this study indicate that PFOS may bring about multiple levels of toxic effects on the gill of P. major.