The invention relates to a preparation method of MMA-BMA-MAA (methyl methacrylate-n-butyl methacrylate-methacrylic acid) ternary copolymer resin, and particularly relates to a method for preparing MMA-BMA-MAA ternary copolymer resin. The method comprises the following steps of: adding deionized water and an auxiliary dispersing agent into a reaction kettle, starting stirring, and heating; adding an initiator, a molecular weight regulator, MMA, BMA and MAA into the reaction kettle, and stirring and heating; after the reaction, adding a dispersing agent into the reaction kettle; continuing the reaction and heating; and washing and sieving to obtain the product ternary copolymer resin. According to the method provided by the invention, by introducing MMA into a polymer molecular chain, the adhesion between a high-paint film and various base materials can be increased; and meanwhile, since the MAA has a similar structure with the MMA and the BMA, a copolymerization reaction easily happens.