The invention relates to the technical field of circuit board metallization treatment, in particular to an FPC flexible board copper plating additive and a preparation method thereof. The additive comprises the following components of, by weight, 2-5 parts of polyether, 0.15-0.3 part of benzyl dithiopropane sodium sulfonate, 0.1-0.2 part of imidazole, 1-3 parts of phenol polyoxyethylene ether, 0.02-0.05 part of yellow dye and 90-96 parts of water. Compared with an existing copper plating additive, the maximum deep plating capacity of the copper plating additive can only reach 150%, and the copper plating additive has 180%-200% deep plating capacity. Under the premise of the same current density and keeping little difference in product performance, by using the additive to carry out the circuit of the flexible board, the electroplating time can be shortened and the production can be improved, and the additive can be suitable for copper plating of rolled copper.