Abstract: Antimicrobial peptides (AMPs) are active substances with antimicrobial or bactericidal functions that can enhance the health of farmed animals. However, it is unknown whether AMPs can improve the intestinal health of grass carp (Ctenopharyngodon idella). In the present study, grass carp was fed with four different diets, including a control group without the antimicrobial peptide Sublancin, and three groups supplemented with Sublancin at doses of 0.9 g/kg (low dose, LD), 1.8 g/kg (middle dose, MD), and 2.7 g/kg (high dose, HD). We utilized a range of analytical methods, including enzyme-linked immunosorbent assay, hematoxylin and eosin tissue staining, transmission electron microscopy (TEM), and Real-time fluorescence quantitative PCR, to investigate the effects of the Sublancin on growth, antioxidant capacity, and intestinal health. The results showed that: (1) Compared to the control group, the final body weight (FBW), weight gain (WG), and weight gain rate (WGR) of grass carp in the MD group increased significantly. Additionally, the specific growth rate (SGR) elevated significantly in both the LD and MD groups (P<0.05). (2) The length and width of intestinal villi, as well as the thickness of the intestinal muscularis propria, exhibited a marked increase in grass carp treated with the antimicrobial peptide Sublancin, while crypts depth in different intestinal segments decreased significantly in the antimicrobial peptide group compared with the control group (P<0.05). (3) The number of intestinal microvilli exhibited a notable increase in both the LD and MD groups compared to the control group. Furthermore, the microvilli in the LD and HD groups were more tightly packed and had a higher density of securely connected electron-dense structures. (4) Compared with the control group, the serum total superoxide dismutase (T-SOD) activity increased significantly in both the LD and MD groups (P<0.05). Meanwhile, the content of serum interleukin-22 (IL-22) in the MD group increased significantly. Additionally, the antimicrobial peptide group significantly decreased the level of serum endotoxin (LPS) and increased the mRNA expression levels of midgut tight junction-related protein genes claudin c and occludin. There was a significant elevation in the mRNA expression level of ZO-2 in the LD group (P<0.05). In summary, the administration of appropriate amounts of the antimicrobial peptide Sublancin can improve the growth performance, enhance antioxidant capacity, strengthen intestinal barrier function, and promote overall intestinal health in grass carp. However, excessive antimicrobial peptides can inhibit the growth of grass carp and decrease oxidative stress capacity and intestinal health. These findings provide theoretical support for the application of antimicrobial peptides in aquaculture.