The invention relates to a method for preparing granular active carbon by using biomass, wherein the granular active carbon is prepared by using biomass as a raw material through hydrothermal carbonization, molding and activation, the biomass comprises corn straw, wheat straw, soybean straw, cotton straw, tobacco straw, rice straw, sawdust, Salix psammophila, water hyacinth or pine branch biomass,and the activator can be zinc chloride, sulfuric acid, phosphoric acid, potassium carbonate, potassium hydroxide and ferric chloride. According to the present invention, corn straw and sawdust are used as the raw materials, and can generate the hydrothermal carbon with strong bonding effect and the hydrothermal liquid with high added value through hydrothermal carbonization, the hydrothermal carbon has a certain bonding effect, and the specific surface area and the pore volume of large-particle active carbon are increased with zinc chloride and ferric chloride, such that the granular active carbon integrating good compressive strength and high adsorption performance is obtained so as to improve the application value.