The invention proposes a method of separating hemicellulose through efficient pretreatment of fibrous biomass materials. Fibrous biomass materials are first pretreated by the slurry method: the fibrous biomass materials are milled through ball mills and sieved to obtain fibrous biomass material powder, and the powder is mixed with water at room temperature to obtain slurry premixed solution of the fibrous biomass material powder. Through regulation and control of the reaction temperature, the time and the water amount, efficient hydrothermal removal of hemicellulose from the fibrous biomass material powder is realized. After the mixed solution is filtered, the filtrate is subject to gradient sedimentation to separate hemicellulose with low dispersity, and the sediment is then dried before storage. After the filter residue is mixed with polar aprotic solvent and water, and heated and stirred in a pressurized reactor, high-purity cellulose is obtained through filtering and separation.