An adhesive used for moisture-proof fiberboards is disclosed. The adhesive comprises following raw materials by weight: formaldehyde, urea, resorcinol, oxidized starch, flour, polyvinyl acetate, an acid solution and an alkali solution. Beneficial effects of the adhesive are that comprehensive modification is performed through adding a small amount of the oxidized starch, the polyvinyl acetate and the resorcinol, curing time of the adhesive in fiberboard hot pressing is shortened, adhesive strength is increased, obvious characteristics including enhanced waterproofness, high mechanical strength, good dimension stability, and the like of the fiberboards are achieved, and problems that fiberboards under high-moisture environments are liable to expand and deform and strength is poor, and the like are overcome.