Ulcerative colitis is a chronic inflammatory bowel disease characterized by mucosal barrier disruption, dysregulated immune responses, and gut microbial imbalance. However, limitations of current therapies highlight the need for safe probiotic interventions. This study evaluated the preventive effects of fermented yak milk-derived Lacticaseibacillus paracasei CD12-1 on dextran sulfate sodium (DSS)-induced colitis in mice. The results showed that DSS caused body weight loss, an increased disease activity index, colon shortening, and severe histopathological injury. CD12-1 alleviated these abnormalities with differential effects across doses. The low-dose treatment produced the most comprehensive improvements in histopathological damage, goblet cell abundance, tight junction integrity, inflammatory cytokines, and short-chain fatty acids, whereas the high dose more effectively attenuated body weight loss. CD12-1 increased colonic ZO-1 and Occludin expression, reduced IL-1β, IL-6, and TNF-α levels, and increased IL-10. The low dose also elevated acetate and butyrate levels. Gut microbiota analysis showed that CD12-1 was associated with changes in the relative abundances of Lactobacillus, Bifidobacterium, Allobaculum, Akkermansia, and several inflammation-associated taxa. Correlation analysis associated Lactobacillus and Allobaculum with milder disease and improved barrier-related indicators, whereas Bacteroides and Sutterella were associated with greater disease severity and inflammation. Overall, CD12-1 alleviated DSS-induced colitis by improving intestinal barrier integrity and inflammatory homeostasis, accompanied by changes in microbial composition and short-chain fatty acid production.