The invention relates to a process method for synthesizing large-particle-diameter zinc borate. The process method comprises the steps of adding water, heating, adding boric acid, stirring until boric acid is completely dissolved, slowly adding zinc oxide and reacting for 4-10 hours, and immediately carrying out centrifugal drying, so as to obtain large-particle-diameter and high-boron-content 3.5 hydrated zinc borate. A preparation method of 2ZnO.3B2O3. 3.5H2O zinc borate has the beneficial effects that the process is simple, the reaction conditions are mild, the production of by-products and wastewater is avoided, mother liquid can be recycled, and the loss of boron is avoided; the yield is high, the cost is low, the three wastes are avoided, and the product quality is extremely high; and the production cost of a workshop is low, and prepared zinc borate has a good effect in the field of corrosion resistance processes of woods.