The invention discloses a high-barrier fast-curing solvent-free polyurethane adhesive and a preparation method thereof, and belongs to the field of preparation of polyurethane adhesives. The adhesive is composed of a component A and a component B, the component A and the component B are used in cooperation according to the functional group molar ratio of-NCO:-OH = 1.1-1.9: 1, and the component A is a modified isocyanate adduct formed by reaction of modified polyester polyol and polyisocyanate; and the component B is a mixture of castor oil derivative polyol, modified polyester polyol and polyether polyol. According to the adhesive, through selection of raw material monomers, design of a molecular chain structure and a specific molecular weight size and regulation and control of a molecular proportion, higher reaction activity and excellent water and oxygen blocking performance are achieved. The high-barrier fast-curing adhesive is excellent in peel strength and can meet the actual application process requirements of the adhesive, so that the high-barrier fast-curing adhesive can be widely applied to the field of composite flexible packaging.