Xylan from corncobs was obtained by the method of high temperature pretreatment and ultrasound-assisted alkaline hydrolysis. The extraction process of xylan was optimized by orthogonal experiment on the basis of single factor.The result indicated that the optimal parameters were as follows: the concentration of sodium hydroxide 7%, ratio of solid to liquid 1∶25, ultrasonic power 250 W, ultrasonic time 40 min, ultrasonic temperature 60 ℃. Under the optimized conditions, the yield of reducing sugar and total sugar in hydrolysate was 64.10 mg/g and 100.14 mg/g of raw corncobs, respectively. The yield of xylan was 9.01%.