Objective To study the effect of waterbased drilling fluid (from a drilling platform in Nanhai district, China) exposure on the activity of antioxidant enzyme in Mugilogobius chulae.Methods The SOD and CAT activity of 3-month-old juvenile Mugilogobius chulae were tested by reagent kit method during the exposure at 19.75 to 158 mg/L concentration. Results The SOD activity of 3-month-old juveniles was not sensitive during the exposure. At the first day of exposure, the SOD activity was slightly inhibited at the lowest concentration (P >0.05). However, the activity was inducted (12.4% to 25.5%, P >0.05) in all treatment at the second day. On the 7-day exposure, the change of SOD activity was limited. The activity of CAT showed high sensitivity to the drilling fluid. During the first day of exposure, the CAT activities of each treatment were inducted and they had a concentration effect. The inductivity of the highest concentration even reached to 97% (P < 0.05). Then, the inductivity slowed down to 58.4% to 89.1% at the 7th day.Conclusion CAT, which shows an apparent sensitivity, meets the important requirement as a biomarker, and is expected to play a role in practical application in the future.