Abstract: The aim of this experiment was to investigate the effects of oleanolic acid supplementation in high cotton rapeseed meal feed on the growth, serum biochemistry, antioxidant, and immunity of grass carp (Ctenopharyngodon idella). Four hundred and eighty grass carp weighing about 50 g were selected and randomly divided into three groups with four replicates of 40 fish each. These groups included the control group, the high cotton rapeseed meal group, and the high cotton rapeseed meal group supplemented with 500 mg/kg oleanolic acid, which were labeled as CON, S1, and SQ2, respectively, with a culture period of 56d. The results showed that: 1. Grass carp in the S1 and SQ2 groups exhibited significantly lower FBW, WGR, and SGR compared with the CON group (P<0.05), with the S1 group showing significantly higher FCR (P<0.05). 2. FBW, WGR, and SGR were significantly higher (P<0.05), and FCR was significantly lower (P<0.05) in grass carp in the SQ2 group compared to the S1 group. Additionally, compared to the CON group, serum GOT, TBA, T-CHO, and hepatic MDA levels were significantly higher (P<0.05) in the S1 and SQ2 groups. Conversely, levels of serum ACP, IgM, and hepatic CAT, SOD, and GSH were significantly decreased (P<0.05). Furthermore, serum LDL-C levels were significantly higher (P<0.05) in the S1 group, whereas serum complement C3 and liver T-AOC levels were significantly lower (P<0.05). Compared to the S1 group, the SQ2 group exhibited significantly lower levels of serum GOT, T-CHO, and hepatic MDA (P<0.05), and significantly higher levels of serum ACP, IgM, and hepatic CAT, SOD, and T-AOC (P<0.05). 3. Compared to the CON group, hepatic expression levels of gpx1, cat, tgf-β1, and il-15 were significantly down-regulated (P<0.05). Moreover, in the livers of grass carp in S1 group, expression levels of Mnsod and nrf2 were significantly down-regulated (P<0.05), whereas the expression levels of keap1, tnf-α, nrf-κb. and il-12β were significantly up-regulated (P<0.05). Conversely, expression levels of Mnsod, gpx1, cat, nrf2, tgf-β1, and il-15 were significantly up-regulated (P<0.05), while the expression levels of keap1, tnf-α, nf-κb, and il-12β were significantly down-regulated (P<0.05) in the livers of the grass carp in the SQ2 group compared to the S1 group. In conclusion, the high percentage of cotton rapeseed meal in the feed significantly reduced the growth performance, antioxidant capacity, and immunity of grass carp, and induced damage to the liver tissue structure. However, supplementation with oleanolic acid alleviated oxidative damage and inflammatory responses in the liver of grass carp, while improving growth performance, antioxidant capacity, and immunity.