Ulcerative colitis (UC) is a growing health concern in humans, but it can be prevented by using special dietary strategies. Young stems and leaves of Brasenia schreberi (BS) are coated with a special polysaccharide hydrogel (BS mucilage) which can be beneficial for colon health. The aim of this study was to investigate the preventive effects of BS mucilage against UC in a DSS-treated mouse model. Although containing only 0.3% solid content, our research showed that BS mucilage effectively attenuated the disease activity index (DAI) and the spleen index and downregulated IL-1β, IL-18, IL-6 and CAT mRNA levels in DSS-treated mice, which is a promising UC alleviation function. Additionally, BS mucilage also improved the propionate and butyrate levels in mouse feces and alleviated the imbalanced gut microbiota induced by DSS. The abundance of pro-inflammatory and colorectal cancer related bacteria, such as Prevotella, Ruminococcus, Acutalibacter and Christensenella, was decreased by BS mucilage feeding, whereas the abundance of anti-inflammatory and SCFA-producing bacteria including Alistipes and Odoribacter was increased. In conclusion, the current study shows that the daily consumption of BS mucilage could be an effective way to prevent UC in mice, via modulation of gut microbiota.