Refractive index is an important parameter in the method of particle measurement, which is based on forward light scattering with Mie theory. In order to minimize the errors of the particle size measurement, the effects caused by relative refractive index are studied with Mie theory and its Debye series expansion as well as the Fraunhofer diffraction approximation. Numerical calculation reveals that the width of distribution of the scattered light energy shows oscillating properties periodically along with variation of the relative refractive index, which can be described with the empirical expressions. The cause of the scattered light energy distribution changing with relative refractive index is introduced as well. Experimental results show that the forward scattered light energy behaves in a periodical oscillating way along with variation of the relative refractive index and the period of oscillation is π. The inversion results, which are related to the minimum points of oscillating curve, are closer to the nominal value. So, it is necessary to make the width of the distribution of the scattered light energy correspond to the minimum points on scattering light signal oscillation curve by choosing dispersion with proper refractive index in practice.