The invention relates to polyester-polyamide melt-assistant agent, a preparation method and application thereof, which belong to the technical field of composite material interface melt-assist agents. The polyester-polyamide melt-assistant agent is prepared by the following steps: recovering polyethylene terephthalate (PET), monoethanolamine, nylon acid, polycondensation catalyst and heat stabilizer as raw materials; respectively crushing and shifting the recovered PET and the nylon acid; adding the recovered PET, the monoethanolamine and the heat stabilizer in a reaction kettle; heating the reaction kettle to dissolve the PET amine; adding the nylon acid for esterification; and adding the polycondensation catalyst for heating, reduced pressure distillation and polycondensation. The melt-assistant agent is used for improving the physical and mechanical properties of agricultural wastes/thermoplastic plastic composite material. The invention blends the recovered PET and the nylon acid to use, reduces the cost of the raw materials of products and improves the additional value of the products; anhydride impurities in the nylon acid can be effectively utilized; the preparation processis simple and easy to operate; and industrial economy and social benefits are obvious.