The invention relates to a high solid content low-viscosity aqueous polyurethane adhesive and a preparing method thereof. The adhesive comprises polyester polyol. Under the effect of a hydrophilic chain extender and a catalyst, polyester polyol reacts with polyisocyanate to form a polyester polyol-polyisocyanate prepolymer, and aqueous polyurethane dispersoid or emulsion is formed by adding sulfamate, deionized water, accessory ingredient and micromolecular chain extender into the prepolymer. Mainly, according to the method, polyester polyol is formed by adopting glycidyl tertiary carboxylic ester monomer and polybasic acid, binary anhydride and monomer containing epoxy groups to aggregate under the effect of accelerant, polymerization inhibitor and antioxidant. By choosing different monomers to aggregate with glycidyl tertiary carboxylic ester monomer or by changing the mass of every monomer component, polyester polyol resin of different molecular weights is obtained, and usage requirements of products in all fields are met. Before aqueous polyurethane emulsifies, solvent does not need to be added to be subjected to dilution. The high solid content low-viscosity aqueous polyurethane adhesive and the preparing method thereof have the advantages of being flexible in preparation, high in adaptability, large in range of application, low in emulsion viscosity, high in content, free of pollution and the like.