Objective To explore whether excessive intake of free sugar（FS）aggravates colitis in mice and its effects on gut microbiota. Methods A colitis model was established by providing mice with free access to 3.5% dextran sulfate sodium（DSS）for 5 consecutive days. A total of 30 specific pathogen-free male C57BL/6J mice were randomly divided into control group

DSS group

and DSS+FS group（FS

11 g/kg）

with 10 mice per group. From the first day of the experiment

FS was given by gavage every other day

with an equal volume of distilled water given by gavage in the control groups. All the mice were weighed and observed for hematochezia and diarrhea every 2 days. At the 12th day

the feces was collected and then the mice were sacrificed in each group. Enzyme-linked immunosor⁃bent assay was used to measure the content of lipopolysaccharide（LPS）in serum. The length of the colon was mea⁃sured. Intestinal epithelial cell apoptosis was determined by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling. The pathological changes of the colon were observed with hematoxylin and eosin staining. the changes of intestinal flora were determined by 16S rRNA V3/V4 region sequencing with the primers 338F （5 ’-ACTCCTACGGGAGGCAGCAG-3’）and 806R（5’-GGACTACHVGGGTWTCTAAT-3’）. Results An acute colitis model was successfully established. Compared with those in the control group

the mice after DSS treatment developed hematochezia

a reduction in body weight

an increase in serum LPS level

a decrease in colon length

mu⁃cosal shedding

inflammatory cell infiltration

crypt distortion

and increased cell apoptosis. The mice with colitis had significant changes in the composition of gut microbiota. At the phylum level

the two groups with DSS treatment showed significantly decreased abundance of Bacteroidetes and significantly increased abundance of Firmicutes

Ver⁃rucomicrobia

and Proteobacteria. At the genus level

the two groups with DSS treatment showed decreased abun⁃dance of norank_f_Muribaculaceae

Faecalibaculum

Candidatus Arthromitus

and Gordonibacter and increased abun⁃dance of Bacteroides

Akkermansia

Lachnospiraceae_UCG_006

and Escherichia-Shigella. The results indicated the presence of dysbacteriosis. Compared with the DSS group

the DSS+FS group had significantly aggravated changes in the above indicators（P<0.05）

suggesting severer colitis and further imbalanced gut microbiota. Conclusion Ex⁃cessive FS intake can promote the development of colitis in mice and cause significant imbalance of gut microbiota.