The invention discloses a rust transforming agent. The rust transforming agent consists of pre-treatment liquid and post treatment liquid, wherein the pre-treatment liquid is prepared from the following raw materials in percentage by weight: 19 to 22 percent of metal iron powder, 9 to 11 percent of urotropin, 9 to 11 percent of glycerol and 58 to 61 percent of water; the post treatment liquid is prepared from the following raw materials in percentage by weight: 45.5 to 47.5 percent of polyphosphoric acid, 37 to 40 percent of water, 2.5 to 3.5 percent of manganese carbonate, 8 to 10 percent ofacetic acid and 4 to 5 percent of tannin. The preparation method comprises the following steps of: the preparation steps of the pre-treatment liquid, namely getting water in proportion, adding the urotropin into the water in proportion, adding the glycerol into the mixture in proportion, and adding the pure iron powder into the mixture in proportion; and the preparation steps of the post treatment liquid, namely adding the polyphosphoric acid in proportion, adding the water in proportion, adding the acetic acid in proportion, adding the manganese carbonate in proportion, adding the tannin in proportion, and stirring the mixture for 2 to 3 minutes. The using method of the rust transforming agent comprises the following steps of: brushing a relatively thin layer of pre-treatment liquid ontothe surface of rusting iron and steel; spraying the post treatment liquid onto the rusting iron and steel one hour later; and staying the rusting iron and steel for 24 hours to obtain an integrated rust transforming film. The rust transforming agent is convenient to use and the rust-preventing period is long.