The invention relates to a curing agent, namely, 4, 4 (1)-diphenylmethane diisocyanate prepolymer and a preparation method thereof. The method comprises the steps that molten 4, 4(1)-MDI and an organic solvent are added into a reaction vessel, and stirred uniformly, the temperature is raised to 65 DEG C, polyhydric alcohol dehydrated through evacuation is added, to react for 2 to 4 hours at 60 to 100 DEG C, and then an MDI modifier is produced; when being stirred, the mixed liquor of trimethylol propane and the organic solvent is titrated into the reaction vessel for 1 to 4 hour(s), the titration temperature is 60 to 100 DEG C, heat preservation and reaction are performed for 1 to 3 hour(s), then the temperature is raised to 90 to 100 DEG C and the heat preservation and the reaction are performed again for 1 to 4 hour(s), the temperature is reduced, and the organic solvent is added for dilution to appropriate viscosity. The curing agent has the advantages that the application scope is broad, the molecular structure of the polyhydric alcohol can be adjusted according to the requirements of users to produce the prepolymer of polyurethane coating, adhesive, and the curing agent, the storage stability is good, the curing speed is quick, the toxicity is low, the price is low, products with high solid content can be obtained, etc.