Abstract: This study aimed to investigate the effects of adding Gardenia jasminoides Ellis or fermented soybean meal with Gardenia jasminoides Ellis to the diet on largemouth bass (Micropterus salmoides). Referring to the traditional Zhizichi Decoction, Gardenia jasminoides Ellis and soybean meal were mixed at a mass ratio of 3:10 and fermented with Actinomucor elegans for 48h to prepare fermented soybean meal with Gardenia jasminoides Ellis. The experiment included three groups: a control group (CON) without the addition of Gardenia jasminoides Ellis; a Gardenia jasminoides Ellis addition group (GD) with an addition amount of 10 g/kg; and a group (GSPD) with an equivalent amount of Gardenia jasminoides Ellis delivered via fermented soybean meal with Gardenia jasminoides Ellis. Isonitrogenous and isolipidic diets were fed to largemouth bass with an initial body weight of (38.19±0.18) g in a recirculating water system for a 7-week feeding trial. The results showed that there was no significant difference in weight gain rate (WGR) between the GD and the GSPD group compared with the CON group (P>0.05), however, the feed conversion ratio (FCR) of the GSPD group was significantly lower (P<0.05). In terms of liver health indicators, GD group exhibited significantly higher plasma aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), and hepatosomatic index (HSI) compared to CON group (P<0.05), with abnormalities such as cell swelling and nuclear atrophy in the liver. In contrast, compared with the GD group, the plasma AST and ALP levels in the GSPD group were significantly lower (P<0.05), and there was no significant difference in the HSI between the GSPD group and the CON group (P>0.05). Regarding the antioxidant capacity, the activities of catalase (CAT), total antioxidant capacity (T-AOC), and superoxide dismutase (SOD) in the GSPD group were significantly higher than those in the GD group (P<0.05), and the contents of malondialdehyde (MDA) and reactive oxygen species (ROS) were significantly lower than those in the GD group (P<0.05), with no significant differences compared with the CON group (P>0.05). The detection of antioxidant-related genes showed that the relative expression level of the keap1 gene in the GSPD group was significantly lower than that in the CON and the GD group (P<0.05), and the relative expression levels of nqo-1, ho-1, sod, and gpx genes were significantly higher than those in the GD group (P<0.05). The detection of inflammation-related genes revealed that the relative expression levels of pro-inflammatory factor genes nf-κb, il-1β, and il-6 in the GSPD group were significantly lower than those in the GD group (P<0.05), and the relative expression level of the anti-inflammatory factor gene il-10 was significantly higher than that in the GD group (P<0.05), with no significant differences compared with the CON group (P>0.05). In conclusion, the addition of Gardenia jasminoides Ellis or fermented soybean meal with Gardenia jasminoides Ellis to the diet had no obvious effect on the growth of largemouth bass, but fermented soybean meal with Gardenia jasminoides Ellis could improve feed utilization efficiency. A dosage of 10 g/kg Gardenia jasminoides Ellis could damage the liver of largemouth bass, while fermented soybean meal with Gardenia jasminoides Ellis enhanced the antioxidant and anti-inflammatory abilities of the body, playing a protective role in liver health.