A mathematical model of the determination of the iron in water using the method of primary secondary wavelengths was proposed.According to the relation between absorbance (A) and the wavelength of the irradiation light(λ) and the iron(x) in water,a way with high accuracy to determine the iron in water was investigated.The calculation curve is stable and hardly affected by environmental conditions.Experiments with water samples have shown that the detection limit was 0.015μg·ml -1 ,RSD≤3.5%,and the recovery rate was 95.4%～101.6%.
