The invention discloses a biomass formaldehyde eliminating agent and a preparing method thereof and belongs to the technical field of formaldehyde eliminating. The defects that an existing formaldehyde eliminating agent is high in cost, poor in effect and large in contamination are overcome. The biomass formaldehyde eliminating agent is prepared from, by weight, 70-120 parts of biomass raw materials, 440-550 parts of deionized water, 0.4-1.3 parts of glycohydrolase, 0.4-0.9 part of a cleaning agent, 0.3-1 part of a reducing agent, 20-50 parts of absolute ethyl alcohol, 4-8 parts of an anionic surfactant, 3-9 parts of Na2SO3 and 2-6 parts of NaOH. The preparing method comprises the steps that the biomass raw materials are smashed and then added with glycohydrolase for hydrolysis; then, filtering is carried out to remove residues, the cleaning agent and the reducing agent are added, and screening is carried out after standing; finally, absolute ethyl alcohol and the anionic surfactant are added and evenly mixed, then the pH value is regulated, and the biomass formaldehyde eliminating agent is obtained. The biomass formaldehyde eliminating agent is suitable for eliminating formaldehyde on the surfaces of various plates and furniture, and can also be used for eliminating formaldehyde in air.