Interference characteristics of arsenic cadmium coexisting system is researched using spectral interference effect of overlapped spectral line at a wavelength of 228.8 nm. By using even design, mathematical method and computer aided data processing, trace arsenic and cadmium are determined simultaneously by routine flare atomic absorption spectrum, in which the sensitivity of arsenic determination can reach 10 ng/ml. The result is satisfying in determination of trace arsenic in water using this method.