The invention discloses an aqueous acrylic emulsion, an aqueous adhesive for plastic-plastic composite materials and a preparation method thereof, and belongs to the field of preparation of adhesive. The acrylic emulsion is polymerized from 30-50 wt% of acrylic acid hard monomer, 40-60 wt% of acrylic acid soft monomer and 5-10 wt% of acrylic acid functional monomer under the action of water, an emulsifying agent, an initiator, a buffering agent and a pH regulator. The invention also provides an aqueous adhesive and a preparation method thereof. The molecule segment of the film forming matter (acrylate copolymer) in the aqueous acrylic emulsion has various chemical groups, such as -OH, -COOH, -NH and the like; and the chemical groups can form various chemical bonds and hydrogen bonding acting force, and thereby having favorable adhering force for various composite substrates (plastic films, aluminum plating films, metal foils and the like). By properly matching the soft monomer and the hard monomer and adopting the corresponding functional monomer to participate in the copolymerization, the invention enhances the adhesive affinity to the plastic films with low surface tension, thereby enhancing the wetting property of the adhesive on the surface of the plastic films and the combined strength.