For the monodispersion of dust and black carbon particles coated by water in internal mixing state, the validity of equivalent refractive index for the mixture is analyzed using extinction-, absorption-, scattering-efficiency factors and asymmetry parameters. The equivalences for extinction-, absorption- and scattering-efficiency factors are good with the relative errors less than 3%、 3% and 4%, respectively at different radius ratios for the size parameters from 0.1 to 25. The equivalence for asymmetry parameters is relatively a bit poor with the relative errors less than 13%. The complex refractive indices can almost be determined for the radius ratio less than 1/5, which is irrespective of the size parameter. It is easy to obtain the equivalent refractive indices of the internal mixture of particles using the optical parameters except the scattering phase function.