A preparation method of polymeric ferric sulfate comprises the following steps in sequence: step a, providing waste sulfuric acid; step b, adding and mixing: sequentially adding at least one iron sheet, waste sulfuric acid obtained in the step (a), and water into a first reaction tank, wherein the volume ratio of waste sulfuric acid to water is 1:1.5-3, and carrying out reactions to form a ferrous sulfate semi-finished product with a ferrous sulfate content of 4 to 6%; step c, providing a second reaction tank for oxidation, adding the ferrous sulfate semi-finished product, water, and sulfuric acid into the second reaction tank, heating the mixture to a temperature of 30 to 45 DEG C, and adding hydrogen peroxide to obtain polymeric ferric sulfate, wherein the volume ratio of ferrous sulfate semi-finished product to water to sulfuric acid to hydrogen peroxide is 100:1:3:3.