Abstract: To enhance the utilization rate of rice bran resources, a mixed fermentation process using Rhizopus oryzae and Lactobacillus fermentum was employed. Through single-factor and orthogonal experiments, the optimal fermentation conditions for rice bran were established, and the effects of fermentation on the structure and functional activity of dietary fiber in rice bran were analyzed. The results indicated that the optimum production process for fermented rice bran involved a material-to-liquid ratio of 1:2.25, steaming for 60 minutes, an inoculation amount of 10%, an inoculation ratio of 11:9, a fermentation temperature of 37 °C, and a fermentation duration of 42 hours. Under these conditions, the resulting fermented rice bran exhibited a slight attachment of white mycelium on its surface, accompanied by a subtle aroma reminiscent of alcohol. The texture was well-balanced, offering an appropriate degree of softness and hardness, with a balanced ratio of sourness and sweetness. The sensory evaluation yielded a score of 9.2±0.1, which was superior to that obtained from non-optimized processes. Compared to pre-fermentation levels, the contents of soluble dietary fiber (SDF) and insoluble dietary fiber (IDF) in rice bran increased by 35.1% and 6.3%, respectively; additionally, the molecular weight rose from 2.39×10⁵ g/mol to 2.84×10⁷ g/mol with a broader distribution range observed post-fermentation. Monosaccharide composition analysis revealed that fermentation introduced galacturonic acid into SDF while altering its monosaccharide composition ratio. Infrared spectroscopy results indicated that fermentation induced changes in the molecular structure of SDF. Furthermore, fermentation enhanced water-holding capacity, oil-holding capacity, cholesterol adsorption capacity, as well as its in vitro antioxidant activity of SDF. These results demonstrate that mixed fermentation utilizing Rhizopus oryzae and Lactobacillus fermentum effectively improves both functional characteristics and antioxidant activity of SDF derived from rice bran. This finding provides a theoretical support for the food application and functionalization of rice bran.