A combination process of alkali impregnation and refining was used as a pretreatment to improve the production of fermentable sugar. The surface structures and crystallinities of wood samples were characterized to explain the relationships between the pretreatment action and enzymatic efficiency. After refining, the reducing sugar contents in hydrolyzates were analyzed by UV-Vis and HPLC. The results showed that the enzymatic efficiency could be improved by the combined pretreatment, due to the increment of specific surface area and the release of more free hydroxyls. Comparing to the sodium hydroxide and deionized water, the impregnation with magnesium hydroxide had low refining energy consumption and high yield of reducing sugar (glucose and xylose) in enzymolysis process, where about 560 kWh/t of the energy was saved in refining, and the yield of the reducing sugar was as high as 91.53%. And the enzymolysis could be improved by a certain amount of magnesium ions. (C) 2013 Elsevier Ltd. All rights reserved.