A content detecting method of Fe<3+> in a water environment by a colorimetric method is disclosed. The content detecting method includes: preparing a developer 4-(4-hydroxy-1-naphthylazo) benzenesulfonic acid dye, preparing a standard solution of the developer, preparing a standard solution of Fe<3+>, preparing an environment water sample, detecting the content of the Fe<3+> in the sample to be detected, and the like. Corresponding measures are provided for guaranteeing the steps. The method can achieve real-time detection of the content of the Fe<3+> in the water environment under acid or neutral pH conditions. The developer has characteristics of good water solubility, high stability, large absorptivity, high analysis sensitivity and good reproducibility. In addition, visible light having a wavelength of 478 nm used in the content detecting method is harmless to organisms. The content detecting method could be used for the fields of biological immunoassay, detection, and the like.