Abstract: In a weak acid buffer solution of HAC-NaAC, when cetylpyridinium bromide(CPB) is mixed with a dyestuff ethyl eosin(EE) after anionic surfactant is added, the colour of the soluotion becomes deeper, and the max absorbtion wavelength is at 516 nm. At this wavelength, the absorbence enhancement is proportional to the concentration of the anionic surfactant(AS). Based on it, a photometric method for determination of anionic surfactant in water was established. At the max absorbtion wavelength, three anionic surfactants, sodium dodecyl benzene sulfonate(SDBS), sodium lauryl sulfonate(SLS), and sodium dodecyl sulfate(SDS), in the ranges of 0~2.05×10^-5 mol/L, 0~2.08×10^-5 mol/L, and 0~2.04×10^-5 mol/L, respectively were found to obey Beer's Law, and the apparent molar absorptivity(ε) of 2.38&×10⁴ L/(mol·cm), 2.82&×10⁴ L/(mol·cm) and 2.98&×10⁴ L/(mol·cm), and the detection limit of 8.42×10^-7 mol/L, 4.56×10^-7 mol/L and 7.95×10^-7 mol/L were obtained respectively. The proposed method has a good sensitivity and selectivity and has been applied to the determination of anionic surfactant in water samples with satisfactory results.