Abstract: A semi-static toxicity test was carried out to study the toxic effects of short-term triazophos (OP) exposure on mussel Perna viridis. The results showed that SOD activity of visceral mass and mantle were inhibited in early exposure stage, and then induced with exposure time progressed. Lower concentrations of OP (0.4 mg · L^-1 and 2.0 mg · L^-1) decreased GST activity in visceral mass and mantle, and then increased as the time went by; GST activity was reduced with higher concentration of triazophos (10.0 mg · L^-1). OP induced the production of GSH; while with the high concentration (10.0 mg · L^-1), it also inhibited GSH producing. Under OP exposure effect, MDA levels significantly increased in visceral mass and mantle. T-AOC was significantly reduced in visceral mass, while T-AOC was induced in mantle. OH radical capacity was reduced by OP, while T-AOC in mantle was induced as OP stress increased. After 72 h recovery in natural seawater, only MDA content in mantle restored. This study shows that the antioxidant defense system of P.viridis can be damaged by short-term triazophos exposure.