Hemicellulose is generally considered a negative factor for enzymatic cellulose hydrolysis, but it was proved to be a positive one for the enzymatic action of delignified pulp in this study. Alkaline wheat straw pulp (AWSP) with different hemicellulose proportion was obtained by treating AWSP with 1 M H2SO4 at 100°C for different time. The soluble fractions of the hydrolyzed mixtures neutralized with Ca (OH)2 together with the insoluble fractions were used for simultaneous saccharification and fermentation (SSF). The ethanol yields reduced while the hemicellulose content of AWSP decreased. The crystallinity increasing after hemicelluloses removal suggested that amorphous hemicellulose interweaving with cellulose prevented cellulose fibers to aggregate compactly and improved the access of cellulases to cellulose. However, the yeast unfermented reducing sugar derived from hemicellulose was inhibitory to SSF.