NOVELTY - Hydrolyzing biomass involves (a) feeding lignocellulosic biomass feedstock into a hydrolysis reactor, adding water to material, and carrying out hydrolysis of mixture, (b) adding water to obtained mixture, and carrying out ultrasonic treatment, (c) carrying out enzyme hydrolysis of solid product, adding hydrolase to substrate, and hydrolyzing product, and (d) collecting resultant hydrolyzate, and recycling hydrolyzate.    USE - The method is useful for hydrolyzing biomass using the device (all claimed).    ADVANTAGE - The method has low water consumption, and low heat consumption, saves energy, is environmentally-friendly, and produces high yield of product. The device is simple in structure, and easy to control.    DETAILED DESCRIPTION - Hydrolyzing biomass involves (a) feeding lignocellulosic biomass feedstock of particle size of not greater than 1 cm into a hydrolysis reactor, adding not less than 20 %mass water to the material, stirring the mixture, and carrying out hydrolysis of the mixture at 100-180 degrees C for 5-60 minutes, (b) adding not greater than 5% water to the obtained mixture, and carrying out ultrasonic treatment at a temperature of 20-100 degrees C for 5-60 minutes, frequency of 20-60 kHz, and power of 50-500 W, (c) carrying out enzyme hydrolysis of the solid product obtained in step (b) by adding 10-30 filter paper unit (FPU) hydrolase per gram of substrate, and hydrolyzing the product for 72 hours until the pH value of the product reaches 4.8, where the high solid content in the substrate is not less than 20%, and (d) collecting the resultant hydrolyzate, and recycling the hydrolyzate. An INDEPENDENT CLAIM is included for device for carrying out the process which comprises hydrolysis tank, post-processing tank, dehydration tank, receiving tank, and an ultrasonic transducer, where the outlet of the hydrolysis tank is connected to the inlet of the post-processing tank, the outlet of the post-processing tank is connected to the dehydration tank, and the receiving tank is connected to the hydrolysis tank through a pipeline.