The effects of Bifidobacterium fermentation on dietary fiber (DF) of prosomillet bran were studied. Firstly, optimal fermentation conditions for extracting soluble dietary fiber were determined through single factor tests and orthogonal experiments. The structural features were evaluated by means of scanning electron microscopy, X-ray diffraction, Fourier infrared spectroscopy, particle size analysis, thermogravimetric analysis, and monosaccharide content. The results showed that some the crystal structure changed from crystal to non-crystal, which was followed by the decrease of thermal stability. The DF's surface loosened and its particle size decreased after fermentation. Meanwhile, DF showed similar spectral characteristics before and after fermentation, but the monosaccharide composition changed. Furthermore, the physicochemical properties were investigated. The fermented DF exhibited higher water swelling capacity, water holding capacity, oil holding capacity. Finally, improvements in glucose adsorption capacity, cation exchange and antioxidant properties were observed. These results indicated that Bifidobacteria fermentation is beneficial to the modification of DF.