Abstract: In order to study the effects of seaweed sulfate polysaccharides on the antioxidant defense system of Pseudomonas aeruginosa（PA）-infected Litopenarus vannamei, in the experiment, the total antioxidant activity of five seaweed sulfate polysaccharides（PE-Ⅰ, PE-Ⅱ, LJP-Ⅰ, LJP-Ⅱ, LJP-Ⅲ） was determined by phosphomolybdenum complex method; BHT was used as the positive control; the diameter of the circle of inhibition of the five seaweed sulfate polysaccharides was determined by the filter paper sheet method; and the half lethal concentration(LC₅₀) of PA for Litopenarus vannamei was also determined. Healthy shrimp were randomly divided into three groups, namely the model group, the experimental group and the control group; with 20 shrimps in each group consisted. The control and model groups were fed with basal diets, while the experimental group was fed with basal diets supplemented with seaweed sulfate polysaccharides, which had the best bacteriostatic effect. After 14 days of feeding, the model and control groups were injected intraperitoneally with 30 μL of PA bacterial suspension(LC₅₀ after 48 h of challenge); the control group was left untreated. Mortality was counted at the 24th, 48th hours, 7th, and 14th day after challenge; the intestinal contents of shrimp in each group were collected 48 h after challenge; and the total number of colonies of PA was counted. The mass concentration of total plasma protein（TP） and phenoloxidase（PO） activity in hemolymph, antioxidant defense enzyme activities such as total superoxide dismutase（SOD）, catalase（CAT）, and glutathione peroxidase（GSH-Px）, and malondialdehyde（MDA） content in hepatopancreatic tissue were determined by using the kits. Shrimp hepatopancreatic tissues were collected and stained with hematoxylin-eosin（H.E.） staining kit for histopathological observation. The results showed that the magnitude of the total antioxidant activity of the six substances was LJP-Ⅱ＞LJP-Ⅰ＞LJP-Ⅲ＞BHT＞PE-Ⅰ＞PE-Ⅱ, in which the total antioxidant activity of LJP-Ⅰ, LJP-Ⅱ, and LJP-Ⅲ was significantly higher than that of BHT（P＜0.05）. The highest diameter of the circle of inhibition of PA was achieved by LJP-Ⅱ, a brown alga kelp polysaccharide with sulfate, at a concentration of 150 mg/mL, which reached（15.05±0.04） mm; therefore, LJP-Ⅱ was selected as a dietary additive to study its effect on the antioxidant defense system of PA-infected Litopenarus vannamei. At an attack dose of 30 μL, the LC₅₀ of PA on Litopenarus vannamei for 48 h was 7.96×10~6 cfu/mL. Compared with the model group, the mortality rate of the experimental group for 14th day after challenge was significantly decreased（P＜0.05）; TP mass concentration and PO activity in hemolymph, and SOD, CAT, and GSH-Px activities in hepatopancreatic tissues were significantly increased（P＜0.05）; and MDA content was significantly decreased（P＜0.05）. PA-infected shrimp fed diets supplemented with fucoidan kelp sulfate polysaccharide LJP-Ⅱ significantly improved the degree of hepatopancreatic tissue, lumen, and tubule damage, and normalized the morphologic structure of the shrimp. The results indicated that the fucoidan kelp polysaccharide LJP-Ⅱ could significantly improve the level of biochemical indexes of antioxidant defense system and reduce the damage of hepatopancreatic tissues caused by PA.