Pretreatment of lignocellulosic materials is a prerequisite to facilitate the disruption of the natural recalcitrance of their carbohydrate-lignin shield and to allow enzymes to easily access the crystalline cellulose surfaces. Recently, pretreatment of ionic liquids (ILs) has been widely studied as a promising pretreatment technique; however, it is too expensive to be commercialized. In this study, an efficient acid-catalyzed aqueous IL pretreatment process was developed to optimize the total sugar conversion of pretreated biomass and to reduce IL usage. The experimental results demonstrated that the total sugar conversion was raised to 92.7 % with the synergistic effects of IL (1,3-dimethylimidazolium dimethylphosphate, [MMIM]DMP) and dilute hydrochloric acid (HCl) under pretreatment conditions of 110 °C for 2 h, compared to the conversion of only 27.3 % obtained with untreated corn stover. Moreover, the addition of the inorganic acids, especially HCl, to the IL pretreatment was found to not only significantly destroy the crystalline structure of cellulose in corn stover, promoting the conversion of cellulose and hemicellulose to monomeric sugars, but also provide an opportunity to reduce the usage of expensive IL solvents.