By using an optical fiber coupling structure, a new particle measuring method through introducing Lorentz classic dispersion formula and multi-wavelength processing was proposed to amend the scattering influence of a sample cell wall and the change of the refractive index. Firstly, the theory of total light scattering for the particle measurement was introduced, and the problems existied in the sample cell and measurement method were studied. Then, the Lorentz dispersion formula was introduced to process the refractive index changes caused by dispersion and the multi-wave length processing was taken to obtain the average particle size and particle size distribution. By establishing an object function, a method based on the genetic algorithm was proposed to implement the inversion of particle refractive index. A new sample cell was designed and manufactured with the end face coupling of optical fiber, and the particle sample dripping placed the end face between two optical fibers was scanned by a spectrometer in multi-wavelengths.Finally, through the algorithm of the computer, the average particle size and particle size distribution were measured. The simulation and experiment result indicates that the average particle size and particle size distribution can be well determined by this method in the case of unknown refractive index.Furthermore, this method has advantages of reliability and stability. When a 2% random noise was put in the transmission extinction measurement values at two wavelengths, the inversion errors of average particle size and refraction index are less than 1% and that of the particle size distribution is less than 5%. It can meet the needs of national standard for mean errors and repeatability errors.