Objective To investigate the protective effect of lycium barbarum polysaccharide (LBP) on ulcerative colitis and its regulatory effect on the intestinal flora. Method 20 specific⁃pathogen free (SPF) and 20 germ⁃free (GF) C57BL/6 healthy male mice were respectively divided into dextran sulphate sodium (DSS) group (n=10, SPF), LBP group (n=10, SPF), GF (DSS) group (n=10) and GF (LBP) group (n=10). The model of ulcerative colitis was established by discontinuously drinking 2% DSS for 7 days with three cycles, totally for 54 days. DSS and GF (DSS) groups were given a normal diet, while LBP and GF (LBP) groups were given 1% LBP dietary supplement throughout the whole process. Finally, the UC phenotype (body weight, colon length, disease index score and myeloperoxidase activity) were assessed in each group. And the feces of mice were collected by stress defecation method, and the composition of intestinal flora was analyzed by 16S rRNA high⁃throughput sequencing. The content of short⁃chain fatty acids in feces was detected by gas chromatography⁃mass spectrometry. Result LBP exerted a significant effect against UC, as indicated by increased body weight and colon length, and decreased disease activity index scores and myeloperoxidase activity. But the protective effect of LBP on UC was reversed in GF mice. The abundance of some probiotics, such as Lactobacillus [(0.55±0.38)×10⁃2 vs (3.83±1.78)×10⁃2, P=0.009], unclassified Lachnospiraceae [(1.84±0.42)×10⁃2 vs (3.90±1.36)×10⁃2, P=0.023], Butyricicoccus [(0.03±0.04)×10⁃2 vs (0.11±0.01)×10⁃2, P=0.007], unclassified S24⁃7 [(9.91±1.44) ×10⁃2 vs (28.90±15.68) ×10⁃2, P=0.004], increased obviously with LBP treatment. In addition, LBP significantly increased the content of butyric acid in the feces of mice with UC [(3.96±0.78) ng/mg vs (7.44±1.67) ng/mg, P=0.022]. However, LBP did not increase the content of butyric acid in GF mice [(6.02±0.13) ng/mg vs (6.05±0.11) ng/mg, P=0.150]. Conclusion LBP can alleviate DSS⁃induced UC by regulating the composition of intestinal flora and promoting the production of butyric acid.