The potential benefits of insoluble dietary fiber (IDF) in the regulation of lipid metabolism have been reported in large prospective cohort studies although the molecular regulatory mechanism is still unclear. Okara is a by-product obtained during soybean processing for soy milk and soybean curd (tofu), which is rarely utilized and can be a cheap potential dietary fiber (DF) resource. In this study, the structure and physicochemical properties of insoluble dietary fiber (SIDF) extracted from okara were characterized, and the prebiotic effects on fat metabolism were investigated in vivo. The results showed that the main monosaccharides of SIDF (90.50%) identified were galactose, arabinose, xylose, rhamnose and glucose. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD) analyses suggested that SIDF had a loose and porous structure, polysaccharide functional groups, and a typical crystalline cellulose I structure. In addition, SIDF had ideal oil-adsorption capacity (OAC; 7.95 g g-1) and significantly improved cholesterol adsorption (11.14 mg g-1) at pH 7.0. In vivo, IDF supplementation reduced the serum lipid levels and inhibited hepatic fat accumulation. Additionally, SIDF administration improved hepatic steatosis by stimulating lipolysis via upregulation of PPARα, CYP4a10 and CPT1a. This is the first systematic study on the composition, structure, physicochemical properties, adsorption function and biological effects of SIDF. The above results show that SIDF could be used as an ideal functional ingredient in food processing as well as play a positive role in improving the added value of okara and promoting its comprehensive utilization.