Birch wood chips were treated in ultrasonic-assisted ball milling hydrolysis with dilute citric acid at room temperature and atmospheric pressure in order to develop an efficient conversion process for the production of fermentable sugars from woody biomasses using wet ball milling hydrolysis system. Wood lignocellulose was easily hydrolyzed during ballmilling treatment into reduced sugars with a total yield of 130 mg/g wood. The concentrations of several major substances in the hydrolyzate were discussed. Corresponding structure transformations before and after milling were analyzed by means of X-ray diffraction (XRD), and UV spectroscopy clearly indicated that this combined treatment could enhance the crystalline and chemical structure changes of wood lignocellulose during ball-milling.