The effect of non-spherical particles shape on particle measuring method based on multi-wavelength light extinction method is investigated. According to the generalized Mie scattering theory, the extinction section and extinction coefficient of spheroidal particles with different major and minor axes under different incident angles are calculated by numerical simulation, and they are compared with the extinction characteristics of equivalent spherical particle. The extinction spectrum is calculated by combining with Lambert-Beer law, and the particle size of the spheroidal particles is inversed by regularization algorithm. Then the influence of the shapes of the spheroidal particles on particle size measurement result is analyzed. The results show that the change of spheroidal particle shape will bring significant deviation to the inversed particle size result compared with spherical assumption. In this paper, relative deviation can exceed 90% for submicron particles under shape parameter a/b of 5.