The invention discloses a wood adhesive prepared by degrading soybean protein by utilizing strong basicity and a preparation method thereof, and relates to a soybean protein-based wood adhesive and a preparation method thereof. The invention solves the problems of low content of the soybean protein, high moisture content and large viscosity existing in conventional soybean adhesives or soybean protein adhesives. The product is prepared by performing a degradation reaction of defatted soybean protein powder or soybean protein isolate, strong base and water, then using strong phosphoric acid to adjust the pH value of the obtained solution, and then adding dialdehyde into the obtained solution to mix the solution evenly. The method comprises the steps of: 1, weighing; 2, preparing strong base solution, heating the strong base solution, adding the defatted soybean protein powder or the soybean protein isolate into the strong base solution under stirring, and then cooling the strong base solution to a room temperature after heating reflux; and 3, using the strong phosphoric acid to adjust the pH value, adding the dialdehyde into the solution, and stirring the solution evenly. The adhesive has the advantages of high content of the soybean protein, low moisture content and low viscosity, and can be used for preparing plywood, flakeboards and medium-density fiberboards.