Lactobacillus plantarum is defined as beneficial bacteria for produceing lactic acid, short-chain fatty acids, antimicrobial peptides, and other active substances through fermentation, and has probiotic effects such as alleviating oxidative damage to cells, regulating immunity, and promoting growth. However, due to the prolonged high-temperature environment during the processing of aquatic feed, the use of viable bacteria is limited in the field. To investigate the optimal supplementation level of dietary inactivated L. plantarum and its metabolites (LPM) for grass carp (Ctenopharyngodon idella), fish with initial mean weight of (80.47±1.04) g were fed four isonitrogenous and isoenergetic diets which were formulated to contain graded LPM levels (0, 300, 600, 900 mg/kg LPM, respectively) for 6 weeks. The results showed that weight gain rate (WGR) and specific growth rate (SGR) showed a trend of first increasing and then decreasing, and the WGR and feed efficiency reached the maximum value when LPM supplemental level was at 300 mg/kg, increasing by 19.09% and 8.57% compared with the control group (P < 0.05), respectively. Condition factor in LPM supplemental groups was significantly lower than that in the control group, and the viscerosomatic index was significantly reduced by 18.84% in LPM-900 group compared with that of the control group ( P < 0.05). The crude protein content in the whole body, protein efficiency rate, and protein deposition rate reached the maximum in LPM-300 group, which were significantly higher compared with those in the control group by 4.40%, 11.97%, and 7.64% , respectively( P < 0.05). The activities of protease in the liver and intestine of LPM supplemental groups were significantly increased compared with those of the control group ( P < 0.05). When LPM supplemental level was 600 mg/kg, the number, length, and width of intestinal villi of C. idella were significantly higher, and the thickness of the intestinal wall muscle layer was significantly lower than those of the control group (P< 0.05). Dietary LPM had no significant effect on the diversity of intestinal flora ofC. idella (P<0.05). When LPM supplemental level was 300 mg/kg, the abundance ofCyanobacteria, Lactobacillus andClostridium increased significantly (P<0.05). LPM inhibited the proliferation ofAeromonas hydrophila, Vibrio parahaemolyticus, and Nocardia. Dietary LPM had no significant effects on activities of glutamic-pyruvate transaminase and glutamic-oxalacetic transaminase, and the contents of triglyceride and glucose in serum (P>0.05). The contents of total cholesterol, high-density lipoprotein cholesterol, and low-density lipoprotein cholesterol in serum increased with the increase of LPM supplementation level. The contents of total protein, albumin, and alkaline phosphatase activity in serum in the LPM supplemented group were significantly higher than those in the control group (P<0.05). In conclusion, adding LPM at 300-600 mg/kg to the diet improved the growth performance ofC. idella via inhibition of the breeding of harmful bacteria, improving intestinal function, and maintaining the normal function of the liver.