The invention discloses a preparation method of high-adhesion adhesive-free flexible copper-clad plates, and the preparation method comprises the following steps: firstly, dissolving a mixture of a benzimidazole structure containing aromatic diamine monomer (I) and a benzoxazine and benzoxazole structure containing aromatic diamine monomer (II) into N-methyl-2-pyrrolidone (NMP), and then adding an aromatic diamine monomer (III) into the obtained mixture so as to obtain an intermediate polyamide acid; then, adding water-carrying agents dimethyl benzene and chlorobenzene into the intermediate polyamide acid so as to obtain a polyimide solution; and removing the water-carrying agents through steaming, and pouring reaction liquid into methanol so as to obtain a precipitate; and completely washing, filtering and drying the precipitate through the methanol and anhydrous diethyl ether so as to obtain polyimide powder, preparing the polyimide powder into a solution, coating the obtained solution on a copper foil, and in a high-temperature drying tunnel, in the presence of nitrogen, removing solvents so as to obtain a high-adhesion adhesive-free flexible copper-clad plate. A product prepared by using the method disclosed by the invention has the characteristics of high peel strength and no crimping, and a resin layer cladded on a copper foil is excellent in mechanical properties and heat resistance and low in water absorption.