In order to explore the effect of silymarin on relieving low salinity induced stress in the Pacific white shrimp, Litopenaeus vannamei, four experimental diets with silymarin supplementation of 0 (control group), 0.1, 0.2 and 0.4 g/kg were prepared. The effects of silymarin on shrimp growth, immune-related enzymes, microstructure of hepatopancreas and intestinal microbiota were analyzed after juvenile L. vannamei (0.080 ±0.002) g were cultured at low salinity of 3 for 8 weeks. The results showed that: ① Silymarin significantly increased the weight gain rate and decreased the feed coefficient of shrimp at low salinity, but had no significant effects on survival rate, fatness and body composition. ② Silymarin increased the activities of digestive enzymes of shrimp. The activity of amylase in hepatopancreas of 0.4 g/kg group was significantly higher than that of the control group. Dietary 0.2 and 0.4 g/kg silymarin significantly increased the activities of digestive enzymes in intestinal tract of shrimp. ③ Silymarin alleviated the oxidative damage of shrimp caused by low salinity and improved the antioxidant capacity of shrimp. In 0.4 g/kg group, the activities of total superoxide dismutase (T-SOD), catalase (CAT) and reduced glutathione (GSH) in hepatopancreas were significantly increased, but there was no significant change in intestinal antioxidant capacity. The activities of intestinal acid phosphatase (ACP) and alkaline phosphatase (AKP) in 0.2 g/kg group were significantly higher than those in the control group, but the activities of lysozyme and phenoloxidase had no significant change. ④ With the increase of dietary silymarin content, the structure of hepatopancreas of shrimp bacame more complete, the arrangement of hepatic corpuscles was compact, the crosssection of hepatic tubules displayed the shape of regular polygonal star, and contained a large number of secretory cells (B cells) and embryonic cells (E cells). ⑤ The relative abundance of Bacteroides and Gracilibacteria decreased significantly in L. vannamei fed 0.4 g/kg silymarin. The results suggest that silymarin as a feed additive can effectively alleviate the stress caused by low salinity, improve the growth and antioxidant performance in L. vannamei, and have positive effects on the hepatopancreas health and the composition of intestinal microbiota.