The sweet potato residue was fermented with aspergillus niger to extract insoluble dietary fiber (IDF). The effects of fermentation time, solid-liquid ratio and inoculation amount on the extraction yield of IDF were investigated. The result showed that the optimal extraction condition was: fermentation time 120 h, solid-liquid ratio 1∶40 and inoculation amount 6%. Under the condition, the yield of IDF was 24.39%. The water holding capacity and swelling capacity of the dietary fiber were 4.33 g/g and 3.57 mL/g respectively.