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Navel orange peel is a potential source of soluble dietary fiber. In order to make full use of this resource and improve its physiological activities, the effects of single and mixed strains fermentation modifications were compared in this study. The structural, physicochemical, and functional properties of soluble dietary fiber fermented by Trichoderma virideand/or Aspergillus nigerwere investigated. The results showed that mixed fermentation resulted in significantly higher activity of complex cellulase than single fermentation. On the second day, when T. virideand A. nigerwere inoculated at a ratio of 3:1, the activities of carboxymethyl cellulase, β-glucosidase, and filter paper lyase were 1.54, 1.61, and 1.30 times those of single fermentation. Mixed fermentation increased the yield of soluble dietary fiber from 6.63% to 15.73%. Scanning electron microscopy and Fourier transform infrared spectroscopic analyses suggested that soluble dietary fiber produced by mixed fermentation had smaller particle size, lower crystallinity, and more disrupted structure. Moreover, the functional properties of soluble dietary fiber were remarkably improved. After mixed fermentation, the glucose and cholesterol adsorption amounts, and α-amylase inhibition activity increased by 1.51, 1.63, and 1.76 times, respectively. In summary, mixed fermentation was an efficient and environmentally friendly modification method. Modified SDF can be utilized in the food processing industry, expanding the range of by-product applications.