The invention discloses a preparation method of wood vinegar. The preparation method comprises the following steps: (1) refining beech wood blocks; (2) adding the beech powder into a full-automatic continuous extraction distillation machine, and carrying out thermal decomposition at 200-400 DEG C for 3-5 hours in the absence of air; (3) introducing flue gas generated dry distillation into a cooling pipe to perform liquefaction, and separating to obtain a wood vinegar crude product; and (4) distilling the wood vinegar crude product at 110 DEG C under the pressure of 4 MPa for 4 hours, and collecting the condensate, thereby obtaining the wood vinegar. The wood vinegar, carbon powder and water are mixed according to a certain proportion to obtain the feed additive. The feed additive is natural and healthy, is free of hormones and antibiotics, and has the advantages of no toxic or side effect and no drug residue. The wood vinegar can enhance the activity and constitution of the livestock/poultry and aquatic animals, and improve the texture of the livestock/poultry and aquatic animals. The wood vinegar can increase the elasticity of eggs, and enhances the nutrition and quality of meat and eggs. The wood vinegar is capable of obviously enhancing the feed efficiency, shortening the culture cycles of the livestock/poultry and aquatic animals, saving the cost and increasing the income.