The invention relates to a method for preparing a multi-layer PCB (printed circuit board). The method comprises the following steps: providing a plurality of core boards according to the number of layers of the multi-layer PCB, wherein the initial copper thickness of the outward surfaces of the outward core boards is more than the target copper thickness; carrying out film stripping treatment on the plurality of core boards; carrying out oxidation treatment on the plurality of core boards subjected to film stripping treatment; using copper foils to carry out lamination treatment on the plurality of core boards subjected to oxidation treatment; and using etching solution to carry out micro-etching copper thinning treatment on the plurality of core boards subjected to lamination to ensure the copper thickness of the outward surfaces of the outward core boards to be equal to the target copper thickness. The formula of the etching solution comprises divalent copper ions (140-160g/l) and hydrogen chloride (3.65-10.95g/l); and the specific weight of the etching solution is 1200-1400g/l. The method has the following beneficial effects: by optimally adjusting the parameters of the micro-etching copper thinning process, the final copper reduction of the product can be very accurate, the inner layer is produced smoothly, the scrappage is reduced and the production cost is saved.