Abstract: To study the protective effect of probiotics on the intestinal mucosal structure of C. idellus, the protective role of Bacillus subtilis on the intestinal mucosal structure damage by Aeromonas hydrophila infection in C. idellus were randomly divided into three groups: ① The control group: each fish was firstly orally administrated with 0.2 mL of aseptic phosphate buffer solution, and then orally administrated with 0.1 mL of aseptic Phosphate Buffer solution at day 2 and 4; ② The Ah group: each fish was firstly orally administrated with 0.2 mL of isotonic saline suspension containing A. hydrophila (1.0×10⁷cfu/mL), and then orally administrated with 0.1 mL of aseptic phosphate buffer solution at day 2 and 4; ③ The Ah+Bs group: each fish was firstly orally administrated with 0.2 mL of isotonic saline suspension containing A. hydrophila, and then orally administrated with 0.1 mL of isotonic saline suspension containing B. subtilis (1.0×10⁷cfu/mL) at day 2 and 4. Middle portion of midgut were collected every day for 7 days to observe intesti-nal mucous morphology and microfilament cytoskeleton of epithelial cells of C. idellus. The results showed that the pathological characters in the Ah group mainly included degeneration, necrosis and shedding of the intestinal epithelial cells, hemorrhage, edema and thicker of the lamina propria. A large number of inflammatory cells were infiltrated and gap of TJ widened obviously. Microvilli became short, sparse and disorder. Mitochondria became obviously swollen and crest reduced, and endoplasmic reticulum expanded. Microfilament of intestinal epithelial cells appeared green vaporous, and the fluorescence intensity of microfilament gradually weakened, which were significantly lower than that in the control group. The B. subtilis reduced damages of intestinal mucosal, including intestinal mucosa slightly shed; hemorrhage and inflammation phenomenon significantly reduced; Gap of TJ obviously narrowed; the number of microvilli increased; the microvilli arranged regularly; mitochondria had no obvious swelling. The fluorescence intensity of microfilament in the Ah+Bs group was higher than that in the Ah group. The results indicated that B. subtilis reduce the structural damage of intestinal mucosal by A. hydrophila infection.