Micropterus salmoides is one of the most economically valuable fish species in China. Currently, information in the effect of probiotics-supplemented feed on M. salmoides is rare. Present study investigated the impact of supplementary Lactobacillus casei K17 with six treatments on growth performance, gut digestion, absorption and non-specific immune, and gut bacterial biota and metabolomics of M. salmoides. A total of 450 M. salmoides (114 day of age, average 33.0 ± 0.5 g) were randomly allocated into six groups for a 69-day rearing experiments. M. salmoides was given commercial feed supplemented with: 1) live L. casei K17 (1 × 10⁹ CFU/g), 2) live L. casei K17 protected by skim milk powder (10 %), 3) heat-killed dead L. casei K17, 4) its fermentation supernatant, 5) skim milk powder (10 %), and 6) normal saline (0.85 % NaCl), designated as LB, MB, DB, FS, MG, and SG, respectively. MB and DB diets significantly increased body weight and growth rate, but decreased feed conversion ratio of M. salmoides (p < 0.05). These two diets also significantly increased gut amylase and lipase activities and microvilli surface areas (p < 0.05). Comparative bacterial biota analysis revealed distinct bacterial community compositions in M. salmoides gut contents among the six groups, which were remarkably dominated by L. casei in LB (90.0 %), MB (95.5 %), and DB (92.4 %) groups. Occurrences of some pathogenic bacteria were significantly reduced (p < 0.05). Meanwhile, up-modulated carbohydrate and down-modulated amino acid metabolisms were observed in LB, MB, and DB groups (p < 0.05). Comparative metabolomic analysis also revealed a number of notably altered metabolites, which, coupled with increased serum LZM and SOD, improved gut non-specific immune of M. salmoides, particularly in MB and DB groups. The results provided a reference of promising probiotic component L. casei K17 to feed industry.