Abstract: Poly diallyl dimethyl ammonium chloride (PDDA) and poly (sodium 4-styrenesulfonate) (PSS) were used as precursor, and disodium 1-nitroso-2-naphthol-3, 6-disulfonate monohydrate as ferrous ions indicator. A self-assembly film with optical sensitivity to ferrous ions was designed and prepared according to the electrostatic self-assembly principle. The self-assembly process of sensitive film were investigated with UV visible spectrophotometer, and the spectral changes of sensitive film with different content of indicator before and after reaction with ferrous ions was analyzed. The surface topography was observed by Atomic Force Microscopy (AFM).By analyzing the characteristic peaks of sensitive film before and after reaction with ferrous ions, the corresponding relationship between the absorbance changes of sensitive film and concentration of ferrous ions was determined tentatively.