The validity of the equivalent refractive index for describing the external mixture of atmospheric aerosol particles was analyzed by scattering efficiencies, cross sections, and phase functions of monodispersions and polydispersions of the external mixture of two typical aerosol particles with their complex refractive indices of very high and very low imaginary parts. Results show that for monodispersions, the equivalence for the absorption efficiency and phase functions is poor; for polydispersions, the equivalence is stable but with quite large residuals. The retrieved size distribution will have a large bias if the equivalent refractive index is employed in light scattering techniques. It was recommended to use light scattering techniques simultaneously with other techniques to monitor the mixture property of aerosol particles.