The exploration of effective deconstruction of biomass complex structures and mild fractionation into individual components is a profound challenge for the development of biorefinery. Herein, a biomass fractionation process, via treating biomass in various aqueous alcohol solutions with the catalysis of acidic ionic liquids 1-butyl-3-methylimidazolium hydrogen sulfate, was demonstrated to fractionate coir and poplar into cellulose materials with a lignin content as low as 0.95% and lignin with a delignification rate of up to 98%. The participation of acidic ionic liquids into the solvent system greatly multiplied the biomass fractionation efficiency. The analysis on effects of the chemical structure and solubility parameter of alcohols on the delignification efficiency provided a rational and meaningful way to predict and screen solvent for the biomass fractionation process. Lignin in the present study exhibited similar structure with milled wood lignin, and comparable molecular and thermal properties with the conventional organosolv lignin.