Octylphenol (OP), a group of alkylphenol polyethenoxy ethers (APEs), is usually present in polluted water. OP has strong estrogenic activity, is easily concentrated and is difficult for animals to degrade; therefore, it may affect the function of the liver of Misgurnus anguillicaudatus. The antioxidant enzyme system is an important protection mechanism in the body that prevents oxidative damage during metabolism of oxygen free radicals. Although some studies of the toxicity of many phenolic contaminations have been conducted in fish and mammals, few data are currently available regarding the oxidative damage and estrogenic activity of OP in fish. To investigate the damage and activity of M. anguillicaudatus, the median lethal concentration (LC₅₀) of OP for 24, 48, 72 and 96 hours and safe concentration (SC) were calculated by the single factor toxicity test. The LC₅₀ for 24, 48, 72, and 96 hours were 3.26, 2.69, 2.12 and 1.82 mg/L, and the SC was 0.54 mg/L. Based on the SC values, male loaches were exposed to OP at four different concentrations (0.12, 0.19, 0.32 and 0.52 mg/L) and the activities of antioxidant enzymes (superoxide dismutase (SOD) and catalase (CAT)) in the liver were determined at 7, 14, 21 d and 28 d. Additionally, the levels of serum vitellogenin (VTG) were examined by measuring the alkali-labile phosphorus content of the lipid-free protein fraction at 7, 14, 21 d and 28 d. The results showed that the activities of SOD did not differ significantly in response to 0.12 mg/L of OP during 14 d, but that they declined significantly with increased exposure time and concentration, with the lowest activity being observed in response to exposure to 0.52 mg/L of OP for 28 d. The CAT activities showed a similar pattern. When compared to the control group (0 mg/L), the levels of serum VTG increased significantly in response to 0.12 mg/L of OP at 7 d, and increased further with increasing concentration. As the exposure time increased, the total VTG activity in the four concentration groups also increased, with the maximum value being observed in response to exposure to 0.52 mg/L of OP for 28 d. These findings indicated that OP had high toxicity toward M. anguillicaudatus and that moderate and high concentrations of OP were able to induce oxidative stress and exert a significant negative impact on SOD and CAT activities of M. anguillicaudatus. Moreover, the inhibition intensified remarkably as the exposure concentration and time increased, resulting in the hepatic defense function of antioxidant enzymes being reduced. This, in turn, caused excess reactive oxygen species (ROS) accumulation, which can lead to oxidative damage of M. anguillicaudatus. The serum VTG could be induced in male loaches treated by OP and the induction intensified greatly with exposure, indicating that OP possesses estrogenic activity that intensified as the exposure concentration and time increased, leading to damage to the reproductive system of M. anguillicaudatus. These results indicate that the estrogenic effect of OP may be closely correlated with oxidative damage in M. anguillicaudatus.