A natural triploid Qihe crucian carp, Carassius auratus, is an important commercial fish, with the twice thickness of common crucian carp in the back. Hypoxia always produces the important effects on fish physiological and biochemical function and immune protection, and assessing the effect of poly I:C stimulation on antioxidant defense in fish under hypoxia is helpful to understand the oxidative stress and immune ability, as well as the susceptibility to pathogen in fish under hypoxia. The experimental fish, with the body weight of (23±2) g, and the body length of (11±1) cm were cultivated at different dissolved oxygen levels (6, 4, 2 and 1 mg/L) respectively for 7 days. Then, each fish was injected 2 mg/mL poly I:C 100 μL in the treated group and was injected 0.75% NaCl 100μL in the control group. The activities of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and the mRNA relative expression of Cu/Zn-SOD, Mn-SOD, CAT and GPx, as well as the content of malondialdehyde (MDA), were measured after injection at 0, 12, 24, 48, 96 and 168 h. The activities of antioxidant enzymes and the levels of mRNA expression in hepatopancreas of fish, under hypoxia for 7 days, were more significantly increased at DO 4 mg/L than at the normal DO 6 mg/L. It implies that fish can reduce the adverse effects at low oxygen level to a certain degree. The activities and the mRNA relative expression of SOD, CAT, GPx in hepatopancreas were more significantly decreased at DO 1 and 2 mg/L than at 6 mg/L. It was suggested that the capacities of antioxidant defense decreased under hypoxia and showed that oxidative stress occurred when the generation rate of ROS exceeded the removal rate, and lead to the accumulation of MDA, a lipid peroxidation product, in hepatopancreas. Therefore, the content of MDA increased with the decreasing of dissolved oxygen concentration. After poly I:C stimulation in hypoxia, antioxidant enzyme activities and mRNA relative expression levels were significantly reduced, compared to the corresponding control, at 48, 96 and 168 h. It is indicated that antioxidant defense ability was reduced in hepatopancreas of C. auratus after poly I:C infection in hypoxia and then caused serious oxidative stress. MDA content was significantly higher in the treated group than that in the control group. It was implied that oxidative stress was intensified in hepatopancreas of C. auratus after poly I:C injection in hypoxia. In this study, it is demonstrated that the antioxidant defense ability decreased under hypoxia and caused oxidative stress. Pathogenic analogues poly I:C stimulation under hypoxia condition could further weaken antioxidant defense ability and aggravate the degree of oxygen stress. In the cultivation of C. auratus, when DO decreased to less than 4 mg/L, dissolve oxygen should be supplemented and increased to maintain the normal level of antioxidant defense in fish.