Abstract: In order to investigate the repairing effects of porcin β-defensin-2（pBD-2） on heat stress-induced morphological and structural damage and expression of tight junction proteins in mouse small intestine, in the experiment, thirty-six 1-month-old Kunming mice were randomly divided into a blank control group, a heat stress group, and a heat stress+pBD-2 group; with 12 mice in each group. The blank control group and heat stress group were given saline by gavage at 10 μL/g of body weight per mouse per day; the heat stress+pBD-2 group was given pBD-2 by gavage at 0.01 μg/g of body weight per mouse per day. After gavage, the heat stress group and the heat stress+pBD-2 group were placed in a 40 ℃ incubator for 1 h. After removal, they were kept at room temperature together with the blank control group. The groups were treated in the same way for a total of 28 d, and were killed by breaking their necks on the 28th day of treatment. The behavior of each group was observed before gavage every day, and the feed intake was recorded and weighed; at the end of the experiment, the average daily feed intake and average daily weight gain of each group were calculated; and the abdominal cavity of the mice was dissected. Duodenum, jejunum and ileum tissues were taken for paraffin section making and H.E. staining respectively, to compare the ratio of height of intestinal villi to depth of crypts and the number of goblet cells per unit area in each group; and the tight junction proteins, ZO-1, Claudin-3 and Occludin, were detected by immunohistochemistry and analyzed for their relative expression. The results showed that during the trial, the blank control group had no unusual behavior; the heat stress group had increased exploratory behavior and was anxious and active; and the heat stress+pBD-2 group was quieter than the heat stress group. The average daily feed intake was significantly higher in the heat stress group than in the heat stress+pBD-2 group and the blank control group（P＜0.05）, while the difference between the heat stress+pBD-2 group and the blank control group was not significant（P＞0.05）. The average daily weight gain of the blank control group and the heat stress+pBD-2 group were significantly higher than that of the heat stress group（P＜0.05）; the average daily weight gain of the black control group and the heat stress+pBD-2 group had no significant differences（P＞0.05）. In the blank control group, the intestines were healthy, without redness and swelling, with moderate intestinal contents; in the heat stress group, the intestinal surface was congested with dark red color, redness and swelling, with more intestinal contents; and in the heat stress+pBD-2 group, the intestines were without redness and swelling, with less intestinal contents. The ratio of villus height to crypt depth in the duodenum, jejunum, and ileum was significantly lower in the heat stress group than in the blank control group and the heat stress+pBD-2 group（P＜0.05）; the difference between the blank control group and the heat stress+pBD-2 group was not significant（P＞0.05）. The number of goblet cells per unit area of duodenum and jejunum in heat stress +pBD-2 group was significantly higher than that in blank control group and heat stress group（P＜0.05）, and blank control group was significantly higher than that in heat stress group（P＜0.05）. The number of goblet cells per unit area of ileum in heat stress+pBD-2 group was significantly higher than that in heat stress group（P＜0.05）, and there was no significant differences between heat stress group and blank control group（P＞0.05）, and no significant differences between heat stress group and blank control group（P＞0.05）.The relative expression of ZO-1 protein in the duodenum, jejunum and ileum was significantly higher in the blank control group and the heat stress+pBD-2 group than in the heat stress group（P＜0.05）; the difference between the blank control group and the heat stress+pBD-2 group was not significant（P＞0.05）. The relative expression of Claudin-3 protein in the duodenum and jejunum was significantly higher in the blank control group than in the heat stress group and the heat stress+pBD-2 group（P＜0.05）; it was significantly higher in the heat stress+pBD-2 group than in the heat stress group（P＜0.05）; the relative expression of Claudin-3 protein in the ileum did not differ significantly among the groups（P＞0.05）. The relative expression of Occludin protein in the duodenum was significantly higher in the blank control group than in the heat stress group; however, the difference between the heat stress+pBD-2 group and both the blank control and heat stress groups was not significant（P＞0.05）. The relative expression of Occludin protein in the jejunum and ileum was significantly higher in the blank control group and the heat stress+pBD-2 group than in the heat stress group（P＜0.05）; however, the difference between the blank control group and the heat stress+pBD-2 group was not significant（P＞0.05）. The results suggested that heat stress affected the digestion and absorption function, the morphology and structure of the small intestine, and decreased the expression of tight junction proteins in the small intestine of mice, and that pBD-2 had a better repairing effect on the above effects caused by heat stress.