Based on the classical Mie scattering theory, the influence on the measured results which is caused by the inputted refractive index in data processing of the small-angle forward scattering laser particle sizer is investigated by means of POWELL inversion algorithm. The error would be within 0.002% in the case that the inputted refractive index equals exactly the natural one of the measured particles. Otherwise, when the inputted refractive index is in the range of (2.0,4.0) the magnitude of the error would be much less than in the range of (1.1,2.0). So, it comes to the conclusion that the value of the natural refractive index should be inputted in data processing if possible and a value larger than 2.0 should be believably chosen as the inputted refractive index if the natural refractive index of the measured particles is unknown.