The invention discloses a method for preparing activated carbon from wheat straw waste residues obtained through an xylitol preparation technology. The method includes the following steps that 1, the wheat straw waste residues are screened, a phosphoric acid solution and a set amount of boric acid are added and mixed uniformly, and the wheat straw waste residues are soaked at the temperature of 70 DEG C-90 DEG C; 2, the soaked sample is taken out and put into a muffle furnace, and the temperature is kept at 450 DEG C-550 DEG C for 0.5-2 h; 3, the wheat straw waste residues carbonized in the step 2 are ground and cleaned to be neutral; 4, powdered activated carbon can be obtained by drying and grinding the neutral activated carbon in the step 3; 5, the activated carbon obtained in the step 4 is activated with a HCl solution to obtain the activated carbon. The wheat straw waste residues from which hemicellulose is extracted are prepared into the powdered activated carbon through a simple chemical physics method, the powdered activated carbon is used for decoloration and detoxification in the process of extracting hemicellulose from a wheat straw raw material to prepare xylooligosaccharide, and high-value utilization of the wheat straw raw material is achieved.