The complex refractive index of smalll particles is determined from the transmittance spectrum by the use of the Simplified Mie scattering theory and Kramers-Kronig relation. The numerical simulation results for the soot particles with relatively large size show that this method is simple as compared with the existing method. The accuracy of this method is improved and the ranges of wavelength and particle size are expanded. At last the transmittance spectrum of the coal particle cloud is measured by an Infrared spectrometer and its complex refractive index is determined.