The present study developed a combined ball milling-aqueous swelling (CBMAS) pretreatment to accelerate the hydrolysis of corncob. The enzymatic hydrolysis of microcrystalline cellulose carried out in the plates and flasks indicated that the response of enzymatic hydrolysis to CBMAS was quite evident. The fermentable reducing sugars of hydrolysates from CBMAS-pretreated corncob was 59.8g/L, which was 1.3 and 1.7 folds higher than those from diluted acid and alkaline pretreated corncob hydrolysates, respectively. Simultaneous CBMAS pretreatment and enzymatic hydrolysis was also conducted, reducing the processing time from 66h to 28h. The enzymatic hydrolysates from CBMAS-pretreated corncob could be directly utilized as the substrate for butanol fermentation without detoxication. Under the optimal conditions, fermentable sugars in the corncob hydrolysate were completely consumed to generate 9.52g/L butanol.