Berberine (BBR) is an isoquinoline alkaloid, which has been used in the treatment of intestinal mucositis. However, BBR on chemotherapy-induced mucositis in cancer patients remains largely unknown. Here, we investigated the effect of BBR on intestinal mucositis induced by 5-fluorouracil (5-Fu) using rat model. We detected the degree of intestinal mucosal damage and inflammatory response in 5-Fu treated rats with or without BBR administration, and investigated the changes of fecal metabolites and gut microbiota using 1H NMR spectroscopy and 16S rRNA. The mechanism was further explored by fecal microbiota transplantation (FMT). Results showed that BBR treated rats displayed less weight loss, lower diarrhea score and longer colon length in 5-Fu treated rats. Meanwhile, BBR treatment significantly increased the expression of Occludin in ileum and decreased the D-lactate content in serum. Moreover, the expression of IL-1β, IL-6 and TNF-α in ileum were suppressed by BBR treatment. The pattern of fecal metabolism changed obviously after treated with 5-Fu, which was reversed by BBR. Importantly, BBR significantly increased the levels of butyrate and glutamine in feces from 5-Fu treated rats. In terms of gut microbiota, BBR enriched the relative abundance of Firmicutes and decreased Proteobacteria at the phylum level. Meanwhile, BBR increased the propotion of unclassified_f_ Porphyromonadaceae, unclassified_f_ Lachnospiraceae, Lactobacillus, unclassified_o_ Clostridiales, Ruminococcus, Prevotella, Clostridium IV, and decreased Escherichia/Shigella at the genera level. Furthermore, principal component analysis (PCA) showed that fecal transplantation led to changes in fecal metabolites. Fecal transplantation from BBR treated rats had low diarrhea score, reduced inflammatory response in ileum, and relieved intestinal mucosal injury, which may be caused by the increased of butyrate level in fecal metabolites. In conclusion, our study provides evidence that BBR regulates fecal metabolites to ameliorate 5-Fu induced intestinal mucositis by modifying gut microbiota. © 2020 The Authors