A composite modified aqueous polyurethane resin comprises 6-19wt% of diisocyanate, 6-15wt% of a macro-molecular polyol, 2-15wt% of castor oil, 1-5wt% of a micro-molecular chain extender, 1-8wt% of an epoxy resin, 2-10wt% of a hydrophilic chain extender, 1-4wt% of a neutralizer, 0.01-0.1wt% of a catalyst, 5-15wt% of a vinyl monomer, 2-15wt% of a solvent, 0.5-2wt% of a post chain extender, 0.01-0.5wt% of an initiator and 50-60wt% of deionized water. A preparation method of the composite modified aqueous polyurethane resin comprises the following steps: 1, adding the macro-molecular polyol, the castor oil and the diisocyanate to a reaction vessel, stirring, heating to 80-90DEG C, and maintaining the temperature for 2h; 2, adding the micro-molecular chain extender, the epoxy resin, the catalyst, the hydrophilic chain extender and the solvent, and maintaining the temperature at 80-84DEG C for 30min; 3, detecting NCO, cooling to below 40DEG C, and adding the vinyl monomer; 4, adding the neutralizer, stirring at a high speed for 30ming, adding the deionized water and the post chain extender, stirring at a middle speed for 10min, and heating to 80DEG C; and 5, adding the initiator, maintaining the temperature for 2h, cooling to 30DEG C, and filtering to obtain the composite modified aqueous polyurethane. The composite modified aqueous polyurethane resin synthesized in the invention has the advantages of very good water dispersity and very good storage stability.