The efficient production of fermentable sugars is a prerequisite for economic ethanol production from lignocellulosic biomass. A cellulose-enriched fraction was obtained by complete dissolution of ball-milled hybrid poplar wood in dimethyl sulfoxide and lithium chloride (DMSO/LiCl). The cellulose-enriched fraction was mainly composed of glucose and xylose. Spectral fitting analysis of CP/MAS (13)C NMR revealed that the cellulose-enriched fraction had para-crystalline structures with somewhat larger contents of crystalline cellulose І(β) than І(α). The cellulose-enriched fraction was much easier to be converted into mono sugars by cellulases than the untreated sample. Scanning electron microscopy (SEM) indicated that the cellulose-enriched fraction was porous, which could likely explain the high enzymatic hydrolysis efficiency. This study provides a novel pretreatment method based on fractional separation of the three biopolymers via complete dissolution system for enhancement of conversion from ignocellulose to biofuels.