The invention discloses a method for preparing bran water soluble dietary fibers through fermentation. According to the method, rhizopus oryzae is adopted as a single strain for solid-state fermentation. According to the method, the rhizopus oryzae is selected for fermentation of wheat bran to prepare water soluble dietary fibers. The method is simple and practical, the product is high in yield and high in purity, the extraction process of the water soluble dietary fibers in the wheat bran is optimized, and the method establishes foundation for structural identification and physical and chemical analysis on the water soluble dietary fibers.