Based on Mie' s light scattering theory, a method to determine the size distribution of large particles using multi-wavelength light extinction is proposed in this paper. Theoretical analysis and numerical calculations show that the method using multi-wavelength extinction at wavelength between 0. 4 μm-0. 8 μm can expand the upper limit of particle measurement to 300 urn. Numerical simulation results and measurement results of monodisperse polystyrene latex spheres as standard referance material are also given in the paper.