The invention relates to a water-resistant wheat flour adhesive and a preparation method thereof, relates to an adhesive, and in particular relates to a modified wheat flour adhesive for preparing water-resistant plywood. The invention aims to solve the technical problems that wheat flour self cannot be dissolved in water, so that a wheat flour adhesive is poor in water resistance and low in adhesion strength. The preparation method comprises the following steps: I, preparing composite thermal treatment wheat flour; II, stirring the composite thermal treatment wheat flour and a polyvinyl alcohol solution at room temperature, adding polyisocyanates, and stirring continuously at the room temperature till being uniformly mixed, thereby obtaining the water-resistant wheat flour adhesive. The water-resistant wheat flour adhesive is mainly prepared from wheat flour as a main raw material, and the water resistance of the wheat flour is improved through composite thermal treatment; the composite thermal treatment wheat flour is dispersed by using the polyvinyl alcohol solution and is finally cross-linked by using the polyisocyanates, then the water resistance of the water-resistant wheat flour adhesive can be further improved, and the water-resistant wheat flour adhesive which is good in adhesion strength, moderate in viscosity and good in water resistance can be finally prepared.