The aim of this study is to assess the preventive effects of Lactobacillus fermentum HY01 (LF-HY01) to dextran sulfate sodium induced-colitis. We observed the ratio of colon weight to its length, colon pathological changes, and the concentrations of pro-inflammatory factors (IFN-γ, IL-12, TNFα, and IL-6) in serum. We also took account of the protein levels of IκBα, NF-κB p65, iNOS, and COX-2, and we measured the best effects of different doses of Lactobacillus fermentum HY01 (low dose group was 109 CFU/kg·bw, high dose group was 1010 CFU/kg·bw) on dextran sulfate sodium-induced colitis mice. The results were remarkable, suggesting that Lactobacillus fermentum HY01 had significant preventive effects in dextran sulfate sodium induced-colitis; simultaneously, the high dose group showed the best results among other groups. It can effectively alleviate the shortened colon length, reduce the ratio of colon weight to its length, reduce edema, inflammatory cells infiltration, and colon mucosa injury, and play an important role in the down-regulation of concentrations of pro-inflammatory factors (IFN-γ, IL-12, TNFα, and IL-6). Above all, Lactobacillus fermentum HY01 shows promising prevention for IκBα degradation, inhibition of NF-κB p65 phosphorylation cascades, and decreases the protein levels of iNOS and COX-2 as well.