The physicochemical and functional properties of insoluble dietary fiber (IDF) from sea rice bran with different particle sizes were compared. The water-holding, oil-holding, swelling, cation exchange, nitrite adsorption, sodium cholate adsorption, cholesterol adsorption, glucose adsorption, and diffusion inhibition capacities of IDF from sea rice bran prepared via ultrafine grinding were measured. The results showed that as the particle size decreased, the water-holding capacity and swelling capacity of sea rice bran IDF increased. Compared with that of the control group, the effect of IDF-M300 was the most significant, reaching 3.21 g/g and 5.33 mL/g, respectively. The oil-holding capacity decreased significantly, with IDF-M300 showing a value of 3.51 g/g (P<0.05). At glucose concentrations of 50-200 mmol/L, the glucose adsorption capacity of sea rice bran IDF increased significantly. The diffusion inhibition ability of IDF increased with decreasing particle sizes. The cholesterol adsorption capacity of IDF-M300 at pH 2 was significantly higher than that at pH 7, reaching 27.13 mg/g. When the concentration was 300 mg/mL, IDF-M300 showed the highest sodium cholate adsorption capacity and nitrite scavenging capacity at pH 2, which were 67.96 and 9.11 mg/g, respectively. The above results demonstrate that treatment with ultrafine grinding significantly improved the physicochemical properties and functional properties of sea rice bran IDF, providing a theoretical reference for the application of sea rice bran IDF in special foods for patients with diabetes.