Polyamide acid for producing a flexible copper plate coated with no rubber through a two-layer method is prepared by polymerizing three kinds of monomer aromatic diamine at the ratio of (2-6):(2-6):1, wherein the three kinds of monomer aromatic diamine are 3,4'-diaminodiphenyl ether, p-phenylenediamine and 2-(4-amino-phenyl)-5-minobenzimidazole with the CAS register number of 7621-86-5, and the molecular structure of 2-(4-amino-phenyl)-5-minobenzimidazole is shown in the specification. The three kinds of substances are dissolved in an N,N-dimethyl formamide solvent, the mixture is cooled to minus 10-minus 5 DEG C to be reacted for 40-50 h, and thus a polyamide acid solution is generated through polymerization, wherein according to the polyamide acid solution, the solid content ranges from 10 g/100 ml to 15 g/100 ml, the viscosity is larger than or equal to 8000 cp, the number-average molecular weight ranges from 50000 to 76000, and the molecular weight distribution index is smaller than 1.4. The molecular structure of polyamide acid synthesized in the way is matched with the copper expansion coefficient CTE; the specific functional group diamine with an imidazole structure is introduced so that amine on imidazole can form a coordination key with copper, connection with copper can be enhanced, and thus the adhesive force between a finished polyamide acid membrane and copper foil is increased.