The invention relates to a polymerized imidazoline corrosion inhibitor and a preparation method thereof. The corrosion inhibitor is obtained through carrying out esterification reaction on styrene-maleic anhydride copolymer and fatty alcohol to generate a semi-esterified maleic anhydride product, and then reacting residual carboxyl with polyamine. In the total mass of all the reactants, the styrene-maleic anhydride copolymer accounts for 20% to 50%, the fatty alcohol accounts for 10% to 40%, the polyamine accounts for 2% to 20%, a catalyst accounts for 0.001% to 0.5%, butanone accounts for 15% to 80%, and the sum of the mass percents of all the reactants is 100%. According to the polymerized imidazoline corrosion inhibitor provided by the invention, a molecular structure contains a lot of imidazoline groups and hydrophobic long alkyl chains, so that the corrosion inhibition performance can be obviously improved; the preparation method is simple and easy to operate; and the polymerized imidazoline corrosion inhibitor still has excellent corrosion inhibition performance under strongly acidic conditions, and the performance requirement of first-class products in the industrial standard is totally met.