According to the accuracy, stability and fast property of algorithms, the numerical algorithms of the classical Mie scattering and Rayleigh scattering were presented in this paper. Compared the numerical results acquired from the Mie scattering theory and Rayleigh scattering theory by using Matlab with Wiscombe′s numerical results, the results acquired from Matlab programs were also accurate. According to Matlab numerical results, the forward light scattering distribution in a certain angle can be determined by Rayleigh scattering theory when the size parameter x is less than 0.3. The size distribution can also be determined. The algorithm of Rayleigh scattering theory is more efficient than the corresponding algorithm acquired from Mie scattering theory. This simplified method can provide theoretical support for online measuring micro particles when some test objects(the relative refractive index, particle type) are determined.