Objective The effect of acute perfluorooctane sulfonate (PFOS) exposure on the induction of oxidative stress was studied in the livers of swordtails (Xiphophorus helleri Heckel) in order to explore the toxic mechanism of PFOS in fish. Methods Swordtails were randomly divided into five groups, with one control group and four experimental groups (3.5, 7.0, 14.0, and 28.0 mg/l). Parallel experimental groups were also established. The oxidative stress index (malondialdehyde aldehyde, MDA) and superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-PX) activity in fish livers were determined after 12 h, 24 h, 48 h, and 96 h exposure. Results After 12 h exposure to PFOS, the 28.0 mg/l exposure group exhibited significant inhibition in SOD activity. However, in comparison with the control group, there were no significant differences in the other three groups（p > 0.05）. SOD activity was significantly induced in two exposure groups (7.0 and 14.0 mg/l) after 24 h exposure and this upward trend was maintained to 96 h. SOD activity increased during the early hours of exposure and reached a maximal value at 24 h, at which point there was a significant difference with the control group, before decreasing gradually and eventually resuming an upward trend at 96 h. This pattern suggests that such activity may be related to PFOS in the biological body through the elimination mechanism of enterohepatic circulation. Over the same timeframe, CAT activity decreased as the concentration of PFOS increased. Three groups (7.0, 14.0, and 28.0 mg/l) were significantly inhibited at 12 h. CAT activity in each group exhibited slight upward trend, but remained lower than the level observed in the control group after 24 h exposure. After 48 h, each group showed a decreasing trend that continued until 96 h and their CAT activity returned to the level recorded at 12 h. Although the changing trends in GSH-PX and CAT activity were observed to be similar, GSH-PX activity in the 28 mg/l exposure group exhibited significant inhibition at each time point. MDA content decreased slightly at 12 h exposure, indicating the influence of bio-metabolic changes or other physiological factors rather than oxidative stress. However, as the exposure time lengthened, the MDA content of each group continued to increase and reached a maximal value at 96 h. Conclusions The results of this study show that a relative high concentration of PFOS has an apparent effect on antioxidant enzyme activities in swordtail livers. The swordtail in vivo experiments undertaken therefore show that PFOS could induce oxidative stress in the liver and that PFOS-induced oxidative damage is a major route for this toxic effect.