In light scattering particle size measurement method, the results based on the traditional measurement model are easily affected by particle impurities in optical path. On the basis of Mie theory, the new relationship between scattering light energy and particle size distribution was derived by combining the small angle forward scattering method with the polarization ratio method. Two objective functions of the traditional small angle forward scattering method and the polarization ratio method of small angle forward scattering were constructed, and an independent mode algorithm, the artificial fish swarm algorithm(AFSA), was introduced to invert the objective functions. In simulations, uniform spherical particles that obey the unimodal distribution of Johnson-SB were used, and the scattering intensity values of the two target functions were added to the random noise of 5%, 10% and 15% respectively, the results demonstrate that the inversion accuracy, anti-noise performance and robustness of the polarization ratio method are significantly better than those of the traditional small angle forward scattering method.