The invention discloses a method for preparing a water-soluble mold core additive and belongs to the technical field of casting. The method comprises the following steps: respectively dissolving a water-soluble high polymer material and a water-soluble inorganic salt into water, mixing, testing the pH value, dropping an organic acid by using a burette, and adjusting the pH value of the mixed solution to 7, thereby obtaining the additive. Due to adoption of the organic high polymer material in the additive prepared by using the method, a water-soluble mold core has relatively high primary strength; the pH value is adjusted by using the organic acid, so that the problem that a micro-drip ejection head is short in service life is solved; as the water-soluble inorganic slat is used as an adhesive of the mold core, the strength can be improved.