Objective: To investigate the correlation between gut microbiota dysbiosis and histological activity grading in patients with ulcerative colitis (UC). Methods: A total of 120 patients with UC were enrolled. According to the Nancy histological index, patients were divided into a mild activity group (score 2, n=58) and a severe activity group (score 4, n=62). Scores of crypt abscesses, inflammatory cell infiltration, glandular destruction, and composite histological severity were compared between the two groups. 16S rRNA sequencing data were used to quantify the relative abundance of key bacterial taxa: Firmicutes, Proteobacteria, Faecalibacterium, Bifidobacterium, and Escherichia?Shigella. Spearman correlation analysis was performed to assess associations between major taxa and inflammatory indicators, including C?reactive protein (CRP), erythrocyte sedimentation rate (ESR), Mayo clinical activity index (Mayo score), and composite histological score. Results: Compared with the mild group, the severe group had lower hemoglobin and albumin (P<0.05), higher white blood cell and platelet counts, CRP, ESR, and positive fecal occult blood (all P<0.05). Histologically, the severe group showed higher scores for crypt abscesses, inflammation, glandular destruction, and composite severity (P<0.001). Microbiota analysis revealed lower Firmicutes, Faecalibacterium, and Bifidobacterium, but higher Proteobacteria and Escherichia?Shigella in severe disease (P<0.05). Escherichia?Shigella correlated positively with CRP, ESR, Mayo score, and composite histological score (P<0.01). Faecalibacterium showed moderate negative correlations with these indicators (P<0.05), and Bifidobacterium showed mild?to?moderate negative correlations with CRP, Mayo score, and composite histological score (P<0.01). Conclusion: Distinct gut microbiota profiles exist across UC histological activity grades. Severe activity is characterized by enriched pathobionts (Escherichia?Shigella, Proteobacteria) and depleted beneficial taxa (Faecalibacterium, Bifidobacterium). Consistent correlations between bacterial taxa and inflammatory/histological indicators suggest gut microbiota features may serve as potential microbial markers reflecting mucosal inflammation severity in UC, providing valuable references for disease assessment.