The invention belongs to the technical field of graphene preparation, and discloses a method for preparing graphene by using agricultural and forestry waste biomass as a carbon source. The method specifically includes the following steps: (1) Take the pulverized agricultural and forestry waste biomass, add it to a reaction kettle filled with water, undergo a hydrothermal reaction, cool to room temperature after the reaction, and then filter, wash, and dry to obtain a solid biochar; (2) mix the alkali with the biochar obtained in step (1), grind it sufficiently to make it evenly mixed, and heat and calcinate in the presence of protective gas; (3) use acid to prepare the sample after calcining in step (2) Soaking in liquid to remove by-products in the reaction, and then filtering, washing the obtained solid with water until the washing liquid is neutral, and then drying to obtain few-layer graphene. The method has the advantages of simple process, high yield, mild reaction conditions, simple required instruments and equipment, no pollution to the environment, using agricultural and forestry waste biomass as raw materials, greatly reducing production costs, and easy to realize industrial production.