Pretreatment filming agent is mainly composed of a variety of complex agents, dispersants, film-forming agents and surfactants. It can chemisorb with metal ions such as iron and calcium or partially positively charged iron atoms, forming a coordination bond, and then produce a layer of insoluble chelating film covering the surface of metals such as carbon steel. The resistance of the film is large and can isolate the metal from the corrosion medium in the water, thus delaying the corrosion.