A water-soluble polysaccharide (SCP-80-I) was isolated from sweet corncob using microwave-assisted compound-enzyme extraction and column chromatography. SCP-80-I is composed mainly of arabinose, mannose, glucose, and galactose in a molar ratio of 0.369:0.824:10.759:0.333, and has a molecular mass of 18350 Da and β-glycosides linkages in its molecular structure. The preliminary hypoglycemic and hypolipidemic activity in streptozotocin-induced diabetic rats was investigated. Rats were administered daily with 100, 200, and 400 mg/kg SCP-80-I for 21 d. The SCP-80-I increased the rat body mass significantly and reduced the blood glucose level in a dose-dependent manner. The SCP-80-I reduced liver swelling; kidney and pancreas hypertrophy; and total cholesterol, triglyceride, and low-density lipoprotein-C levels significantly; and increased the level of high-density lipoproteins in streptozotocin-induced diabetic rats. These results indicate that SCP-80-I exerts a potential hypoglycemic effect in streptozotocin-induced diabetic rats.